Another milestone achieved with dbx 231s

mbhangui

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An unexpected thing happened (good one).

I was never truly happened with my setup. My chain has over the years undergone lot of permutations and combinations. Being a DIYer have tried plethora of options. Sometimes they improved, sometime they didn't. The most recent improvement was gettinng depth which is recounted here. I always had a good sound stage but getting depth was elusive. But with the help of forum members I did achieve that.

Few things about my setup

AMP, speaker etc.

1) Have an age old Yamaha RX-667 receiver (110 w each channel) which is the one I use for most of my music listening. It is enjoyable and I have to make the least effort to play music This is connected to Polk Rti A9, polk Fxi A6 surround speakers and polk Csi A6 center channel. The cables are standard 12AWG cables
2) Have a tube amp connected to electrostatic pair. Playing music on this is a PITA. I have to remove the cover which protects the electrostats from dust. Wait few seconds for the tube to warm up and it is 20 watts. On few occasions when few drops of CH3COOH has gone down, the power isn't enough. The electrostat are connected using van del hul revolution speaker cable.
3) A DIY Mauro Penasa amp 40+40 watts. This is good, But i have a small heat sink and this amp gets very hot. So this build is waiting for a good sink.
4) Allo volt+ Class D amp (60 + 60) watts. Depending on some whim or fancy, I use this instead of my valve amp to play 2 channel music

To use 3 and 4, I have to manually change the speaker cable connections.

Sources
The most commonly used source is my Allo Piano 2.1 DAC. This I2S dac has DSP and allows me to channel low frequency separately to my subwoofer. This is quite good and musical. The allo Piano DAC is connected to a Allo reclocker which has very accurate clock from two NDK crystals. The Kali Hat is connected to a RPI4 running my own custom Raspian and does everything that moode, volumio, picore, etc can do and does it better because it is running 64 bit OS and 64 bit kernel instead of 32 bit rest like dietpi, volumio, moode, etc are running. Also this doesn't run any web server, etc to give a user friendly UI. I use standard linux, IOS, android clients for mpd on external devices. No client software runs on the RPI. So the entire power of this raspberry PI goes only towards Music. A 4 Tb hard drive with ext4 filesystem is connected to this RPI4 on USB3.0. This RPI4 also runs NFSV4 server.

The other dac which I use is Allo Revolution DAC connected to allo usbridge. The allo uisbridge also runs my own custom raspbian OS and automounts the hard disk of the RPI4 using autofs.

The RPI4 as well as the usbridge run mpd (I have LMS, raspotify, airplay too, but they are not used often).

Both Piano DAC and revolution dac output goes to a DIY Pass B1 buffer preamp, which has a selector switch to select one of 5 inputs. The output of this B1 goes to another self made preamp using Rod Elliot's Preamp. I have a 4 pole selector switch and I can use this switch to direct the output to my AVR, Tube AMP, Mauro Penasa amp or the Allo AMP.

A 2012 mac mini hacked to run Big Sur so that I can run apple music and consume my apple music subscription. The sound from this setup is just meh.. So I have connected my oldest DAC to the mac mini. This is the Xonar Essence Dac. The outpt of this DAC also goes to one of the input of the Pass B1 buffer.. When I want to listen to something that I don't have in my hard disk collection, I use this. The USB input of this dac is connected to a 2 port printer switch. This allows me to share the DAC with two computers. The other computer is a HTPC running openSUSE Tumbleweed and Kodi for primarily movies.

A technics SL-D2 connected to project phono stage, output of which goes to the Pass B1 buffer.

All sources are connected using self made cables using cat6 twisted pairs cannibalized from broken ethernet cables. I have found them to be giving better sound than all commercial cables in the higher frequencies. These cables have very low capacitance.

Graphic Equalizer

Two days back I received this graphic equalizer which was pre-owned by @aeroash listend in the sales section. I had noticed that this has TRS as well as balanced inputs and output. Now only my Xonar Essence has Balanced out. So I connected output of the Xonar to the graphic equalizer and output of the graphic equalizer to the Pass B1 input.

Switched on the mac mini and started playing some Dub genre music. Holy smoke. I was startled. I confirmed that the equalizer controls were all in flat mid position. The only thing was that I had turned on the input gain control to full. It also has ±12 dB buttons for extra gain. But I kept that off. Apple music never had sounded this good. I removed the equalizer and I again got the anemic sound. It left me wondering if the balanced out of the Xonar DAC had caused this effect. Then I connected the equalizer but turned on and off the ±12 dB input gain switch. Regardless of the gain switch I got a stellar performance.

Today morning I took out my soldering iron and made two TRS to RCA cables, so that I could connect the equalizer for each and every music source. And the astounding performance now is heard for all music sources.

The thing is that I'm getting amazing dynamics without actually boosting or bucking any of the frequencies. Can a extra gain from preamp cause so much difference? Also I'm enjoying it with AVR. Tomorrow I will try with my two channel setup, but the sound from the AVR is too good for my ears. Is AVR good for music. In my case I'm loving it. Tomorrow may be another day. Let's see.

IMG_20220425_213250.jpeg
 
Last edited:
An unexpected thing happened (good one).

I was never truly happened with my setup. My chain has over the years undergone lot of permutations and combinations. Being a DIYer have tried plethora of options. Sometimes they improved, sometime they didn't. The most recent improvement was gettinng depth which is recounted here. I always had a good sound stage but getting depth was elusive. But with the help of forum members I did achieve that.

Few things about my setup

AMP, speaker etc.

1) Have an age old Yamaha RX-667 receiver (110 w each channel) which is the one I use for most of my music listening. It is enjoyable and I have to make the least effort to play music This is connected to Polk Rti A9, polk Fxi A6 surround speakers and polk Csi A6 center channel. The cables are standard 12AWG cables
2) Have a tube amp connected to electrostatic pair. Playing music on this is a PITA. I have to remove the cover which protects the electrostats from dust. Wait few seconds for the tube to warm up and it is 20 watts. On few occasions when few drops of CH3COOH has gone down, the power isn't enough. The electrostat are connected using van del hul revolution speaker cable.
3) A DIY Mauro Penasa amp 40+40 watts. This is good, But i have a small heat sink and this amp gets very hot. So this build is waiting for a good sink.
4) Allo volt+ Class D amp (60 + 60) watts. Depending on some whim or fancy, I use this instead of my valve amp to play 2 channel music

To use 3 and 4, I have to manually change the speaker cable connections.

Sources
The most commonly used sources are my Allo Piano 2.1 DAC. This I2S dac has DSP and allows me to channel low frequency separately to my subwoofer. This is quite good and musical. The allo Piano DAC is connected to a Allo reclocker which has very accurate clock from two NDK crystals. The Kali Hat is connected to a RPI4 running my own custom Raspian and does everything that moode, volumio, picore, etc can do and does it better because it is running 64 bit OS and 64 bit kernel instead of 32 bit rest are running. ALso this doesn't run any web server, etc to give a user friendly UI. So the entire power of this raspberry PI goes only towards Music. A 4 Tb hard drive with ext4 filesystem is connected to this RPI4 on USB3.0. This RPI4 also runs NFSV4 server.

The other dac which I use is Allo Revolution DAC connected to allo usbridge. The allo uisbridge also runs my own custom raspbian OS and automounts the hard disk of the RPI4 using autofs.

The RPI4 as well as the usbridge run mpd (I have LMS, raspotify, airplay too, but they are not used often).

Both Piano DAC and revolution dac output goes to a DIY Pass B1 buffer preamp, which has a selector switch to select one of 5 inputs. The output of this B1 goes to another self made preamp using Rod Elliot's Preamp. I have a 4 pole selector switch and I can use this switch to direct the output to my AVR, Tube AMP, Mauro Penasa amp or the Allo AMP.

A 2012 mac mini hacked to run Big Sur so that I can run apple music and cosume my apple music subscription. The sound from this setup is just meh.. So I have connected my oldest DAC to the mac mini. This is the Xonar Essence Dac. The outpt of this DAC also goes to one of the input of the Pass B1 buffer.. When I want to listen to something that I don't have in my hard disk collection, I use this. The USB input of this dac is connected to a 2 port printer switch. This allows me to connect the DAC to two computers.

A technics SL-D2 connected to project phono stage, output of which goes to the Pass B1 buffer.

All sources are connected using self made cables using cat6 twisted pairs cannibalized from broken ethernet cables. I have found them to be giving better sound than all commercial cables in the higher frequencies. These cables have very low capacitance.

Graphic Equalizer

Two days back I received this graphic equalizer which was pre-owned by @aeroash listend in the sales section. I had noticed that this has TRS as well as balanced inputs and output. Now only my Xonar Essence has Balanced out. So I connected output of the Xonar to the graphic equalizer and output of the graphic equalizer to the Pass B1 input.

Switched on the mac mini and started playing some Dub genre music. Holy smoke. I was startled. I confirmed that the equalizer controls were all in flat mid position. The only thing was that I had turned on the input gain control to full. It also has ±12 dB buttons for extra gain. But I kept that off. Apple music never had sounded this good. I removed the equalizer and I again got the anemic sound. It left me wondering if the balanced out of the Xonar DAC had caused this effect. Then I connected the equalizer but turned off the ±12 dB input gain switch. Even then I got a stellar performance.

Today morning I took out my soldering iron and made two TRS to RCA cables, so that I could connect the equalizer for each and every music source. And the astounding performance now is heard for all music sources.

The thing is that I'm getting amazing dynamics without actually boosting or bucking any of the frequencies. Can a extra gain from preamp cause so much difference? Also I'm enjoying it with AVR. Tomorrow I will try with my two channel setup, but the sound from the AVR is too good for my ears. Is AVR good for music. In my case I'm loving it. Tomorrow may be another day. Let's see.


View attachment 68989
Haha, glad you are enjoying the EQ. I thought that I saw it somewhere before reading your post.

My Topping TP60 was struggling to play loud on it's own. When I tried a Tube pre with it, it had no problems playing to my levels. Guess, extra gain does help.
 
Haha, glad you are enjoying the EQ. I thought that I saw it somewhere before reading your post.

My Topping TP60 was struggling to play loud on it's own. When I tried a Tube pre with it, it had no problems playing to my levels. Guess, extra gain does help.
After this experience, I think a good preamp is a must. The integrated amps probably have a good preamp and that could be a reason for many folks saying that an IA is better than an AVR. My feel is that if both have the same power and both get equally good source, they should sound the same.
 
An unexpected thing happened (good one).

I was never truly happened with my setup. My chain has over the years undergone lot of permutations and combinations. Being a DIYer have tried plethora of options. Sometimes they improved, sometime they didn't. The most recent improvement was gettinng depth which is recounted here. I always had a good sound stage but getting depth was elusive. But with the help of forum members I did achieve that.

Few things about my setup

AMP, speaker etc.

1) Have an age old Yamaha RX-667 receiver (110 w each channel) which is the one I use for most of my music listening. It is enjoyable and I have to make the least effort to play music This is connected to Polk Rti A9, polk Fxi A6 surround speakers and polk Csi A6 center channel. The cables are standard 12AWG cables
2) Have a tube amp connected to electrostatic pair. Playing music on this is a PITA. I have to remove the cover which protects the electrostats from dust. Wait few seconds for the tube to warm up and it is 20 watts. On few occasions when few drops of CH3COOH has gone down, the power isn't enough. The electrostat are connected using van del hul revolution speaker cable.
3) A DIY Mauro Penasa amp 40+40 watts. This is good, But i have a small heat sink and this amp gets very hot. So this build is waiting for a good sink.
4) Allo volt+ Class D amp (60 + 60) watts. Depending on some whim or fancy, I use this instead of my valve amp to play 2 channel music

To use 3 and 4, I have to manually change the speaker cable connections.

Sources
The most commonly used source is my Allo Piano 2.1 DAC. This I2S dac has DSP and allows me to channel low frequency separately to my subwoofer. This is quite good and musical. The allo Piano DAC is connected to a Allo reclocker which has very accurate clock from two NDK crystals. The Kali Hat is connected to a RPI4 running my own custom Raspian and does everything that moode, volumio, picore, etc can do and does it better because it is running 64 bit OS and 64 bit kernel instead of 32 bit rest like dietpi, volumio, moode, etc are running. Also this doesn't run any web server, etc to give a user friendly UI. I use standard linux, IOS, android clients for mpd on external devices. No client software runs on the RPI. So the entire power of this raspberry PI goes only towards Music. A 4 Tb hard drive with ext4 filesystem is connected to this RPI4 on USB3.0. This RPI4 also runs NFSV4 server.

The other dac which I use is Allo Revolution DAC connected to allo usbridge. The allo uisbridge also runs my own custom raspbian OS and automounts the hard disk of the RPI4 using autofs.

The RPI4 as well as the usbridge run mpd (I have LMS, raspotify, airplay too, but they are not used often).

Both Piano DAC and revolution dac output goes to a DIY Pass B1 buffer preamp, which has a selector switch to select one of 5 inputs. The output of this B1 goes to another self made preamp using Rod Elliot's Preamp. I have a 4 pole selector switch and I can use this switch to direct the output to my AVR, Tube AMP, Mauro Penasa amp or the Allo AMP.

A 2012 mac mini hacked to run Big Sur so that I can run apple music and consume my apple music subscription. The sound from this setup is just meh.. So I have connected my oldest DAC to the mac mini. This is the Xonar Essence Dac. The outpt of this DAC also goes to one of the input of the Pass B1 buffer.. When I want to listen to something that I don't have in my hard disk collection, I use this. The USB input of this dac is connected to a 2 port printer switch. This allows me to share the DAC with two computers. The other computer is a HTPC running openSUSE Tumbleweed and Kodi for primarily movies.

A technics SL-D2 connected to project phono stage, output of which goes to the Pass B1 buffer.

All sources are connected using self made cables using cat6 twisted pairs cannibalized from broken ethernet cables. I have found them to be giving better sound than all commercial cables in the higher frequencies. These cables have very low capacitance.

Graphic Equalizer

Two days back I received this graphic equalizer which was pre-owned by @aeroash listend in the sales section. I had noticed that this has TRS as well as balanced inputs and output. Now only my Xonar Essence has Balanced out. So I connected output of the Xonar to the graphic equalizer and output of the graphic equalizer to the Pass B1 input.

Switched on the mac mini and started playing some Dub genre music. Holy smoke. I was startled. I confirmed that the equalizer controls were all in flat mid position. The only thing was that I had turned on the input gain control to full. It also has ±12 dB buttons for extra gain. But I kept that off. Apple music never had sounded this good. I removed the equalizer and I again got the anemic sound. It left me wondering if the balanced out of the Xonar DAC had caused this effect. Then I connected the equalizer but turned on and off the ±12 dB input gain switch. Regardless of the gain switch I got a stellar performance.

Today morning I took out my soldering iron and made two TRS to RCA cables, so that I could connect the equalizer for each and every music source. And the astounding performance now is heard for all music sources.

The thing is that I'm getting amazing dynamics without actually boosting or bucking any of the frequencies. Can a extra gain from preamp cause so much difference? Also I'm enjoying it with AVR. Tomorrow I will try with my two channel setup, but the sound from the AVR is too good for my ears. Is AVR good for music. In my case I'm loving it. Tomorrow may be another day. Let's see.

View attachment 68989
That’s a wonderful description of your systems and their performance @mbhangui. Your quest for new discoveries in audio is inspiring!
If I am visiting those parts I would love to come for a listen (if convenient for you, of course) The set up with the First watt amp is particularly enticing.
 
That’s a wonderful description of your systems and their performance @mbhangui. Your quest for new discoveries in audio is inspiring!
If I am visiting those parts I would love to come for a listen (if convenient for you, of course) The set up with the First watt amp is particularly enticing.
Sure. Would love to have you anytime you visit. The PASS B1 has been with me since 2012. I had got the kit from Sachin. The cabinet was designed in this very forum and a group buy was initiated for it. The power supply was designed by Om Mishra. I have also the DC B1 kit without the coupling capacitors. Have not been able to get a good cabinet. Some day I will try that and Kuartlotron super buffer, another kit that i got, finished building it long time back but not yet tried because lack of cabinet.
 
After this experience, I think a good preamp is a must. The integrated amps probably have a good preamp and that could be a reason for many folks saying that an IA is better than an AVR. My feel is that if both have the same power and both get equally good source, they should sound the same.
In my experience, preamp is the most critical component in the system. It is one of the reason I am still sticking to my Teac DAC which has an excellent pre built-in.
With modern sources which output clean 2 volts or more, one does not need high gain preamps. If your power amp has input impedance in excess of 100000 ohms (even 50k if good enough), a good passive preamp is the way to go or one can add a good buffer stage before the pre.
 
In my experience, preamp is the most critical component in the system. It is one of the reason I am still sticking to my Teac DAC which has an excellent pre built-in.
With modern sources which output clean 2 volts or more, one does not need high gain preamps. If your power amp has input impedance in excess of 100000 ohms (even 50k if good enough), a good passive preamp is the way to go or one can add a good buffer stage before the pre.
A buffer between the pre and the power amp or buffer before the pre?
 
If you have imedance mispatch between pre and power, a buffer after pre helps. but it also means an extra piece of component and an extra pair of cables
Got it. I will remove Pass B1 from the chain have the output of DAC go to the equalizer and output of equalizer directly to the amp and see.

Right now it is
DAC --> PASS B1 --> graphic equalizer --> passive preamp with selector to chose amp --> amp -->speaker
 
IME - for me - in my setup - the best preamp has been no preamp - just a passive attenuator.

I too have this dbx 231 but was never able to match it with my setup - with EQ or EQ bypassed. I must have taken this piece of equipment on/off several times with my setup and each time i was disappointed. Maybe its time for me to put a "For Sale" post.
 
IME - for me - in my setup - the best preamp has been no preamp - just a passive attenuator.

I too have this dbx 231 but was never able to match it with my setup - with EQ or EQ bypassed. I must have taken this piece of equipment on/off several times with my setup and each time i was disappointed. Maybe its time for me to put a "For Sale" post.
Indeed. I too am surprised. Somehow it has achieved synergy with my amp. Prior to this I had a vintage Kenwood GE 870 equalizer. The only change I did was to insert the DBX instead of the Kenwood. The decay of the instruments is the single most thing that has improved by leaps and bounds. The thing is that I still can't believe what is happening. Yesterday night, I again pulled out the DBX and tried both combinations. Put back the kenwood and also connected the output of DAC directly to the pass B1.
My earlier setup was like this

DAC -> PASS B1 --> Kenwood GE 850 --> passive preamp --> amplifier
now it is like this
DAC --> PASS B1 --> DBX --> passive preamp --> amplifier

Passive preamp has a 2 pole 2 position selector switch which can output to AMP1 or AMP2
PASS B1 has a selector switch to select one of 4 inputs (DAC1, DAC2, DAC3, Project Box Phono output)
AMP1 is my Yamaha AVR
AMP2 is my Cadence VA 1 tube amp

Now I tried 3 DACs. With each and every DAC I'm getting a performance that I never got before. All sliders of the equalizer are in the flat position.
Mac Mini - DAC1 --> PASS B1
RPI4 -> DAC2 --> PASS B1
RPI3 (usbridge) -> DAC3 - PASS B1
DAC1 - xonar essence one
DAC2 - Piano 2.1 I2S connected to RPI4
DAC3- Allo Revolution DAC connected to allo usbridge (basically RPI3 compute board)

Today I will be trying extended session with the Tube amp.
 
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Indeed. I too am surprised. Somehow it has achieved synergy with my amp. Prior to this I had a vintage Kenwood GE 870 equalizer. The only change I did was to insert the DBX instead of the Kenwood. The decay of the instruments is the single most thing that has improved by leaps and bounds. The thing is that I still can't believe what is happening. Yesterday night, I again pulled out the DBX and tried both combinations. Put back the kenwood and also connected the output of DAC directly to the pass B1.
My earlier setup was like this

DAC -> PASS B1 --> Kenwood GE 850 --> passive preamp --> amplifier
now it is like this
DAC --> PASS B1 --> DBX --> passive preamp --> amplifier

Passive preamp has a 2 pole 2 position selector switch which can output to AMP1 or AMP2
PASS B1 has a selector switch to select one of 4 inputs (DAC1, DAC2, DAC3, Project Box Phono output)
AMP1 is my Yamaha AVR
AMP2 is my Cadence VA 1 tube amp

Now I tried 3 DACs. With each and every DAC I'm getting a performance that I never got before. All sliders of the equalizer are in the flat position.
Mac Mini - DAC1 --> PASS B1
RPI4 -> DAC2 --> PASS B1
RPI3 (usbridge) -> DAC3 - PASS B1
DAC1 - xonar essence one
DAC2 - Piano 2.1 I2S connected to RPI4
DAC3- Allo Revolution DAC connected to allo usbridge (basically RPI3 compute board)

Today I will be trying extended session with the Tube amp.
Good to know that the dbx works in your setup. Btw, at what position is your gain control? I need to keep at almost -12 dB when i connected directly from my Marantz BR player and even after that it was quite loud forcing me to use my passive preamplifier.
 
Good to know that the dbx works in your setup. Btw, at what position is your gain control? I need to keep at almost -12 dB when i connected directly from my Marantz BR player and even after that it was quite loud forcing me to use my passive preamplifier.
Aaah. My gain control is at full +12 db. The clip led is off. The output led level indicator, the -10 db led is on and only the 0 db led blinks. I have adjusted the PASS B1 volume control almost at minimum. If I don't do that, the clip indicator turns on.

I had an extended session with my tube amp with 4 pegs of bloody mary and I'm not getting the same bloody synergy as with the AVR. So gone back to listening to the AVR.

I'm still trying to make sense of what is happening.

Yanni, Wayne Shorter, and Jane Bunnet sounded fabulous. Jim Hall not so much

 
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Good to know that the dbx works in your setup. Btw, at what position is your gain control? I need to keep at almost -12 dB when i connected directly from my Marantz BR player and even after that it was quite loud forcing me to use my passive preamplifier.
An update on this. I have this preamp of Rod Elliot's design https://sound-au.com/project88.htm

I am using two LME49720NA op amps and I modified R7 to 4.7k. This gives a gain of 12.9 dB. I removed the graphic equalizer and I'm getting the same dynamics.

If I don't have this gain before the AMP and intead I turn up the volume of the AMP to the same level, I definitely do not get the dynamics.
So it is not the graphic equalizer doing some magic. It is having a high gain of the output of my source before feeding it to the AMP that is giving me the performance.

What could be happening?
 
An update on this. I have this preamp of Rod Elliot's design https://sound-au.com/project88.htm

I am using two LME49720NA op amps and I modified R7 to 4.7k. This gives a gain of 12.9 dB. I removed the graphic equalizer and I'm getting the same dynamics.

If I don't have this gain before the AMP and intead I turn up the volume of the AMP to the same level, I definitely do not get the dynamics.
So it is not the graphic equalizer doing some magic. It is having a high gain of the output of my source before feeding it to the AMP that is giving me the performance.

What could be happening?
Interesting observations.

Would you have an idea what is the overall system gain before you introduced the dbx?

My rough estimate with the B1 in the chain would be:

1/ B1: gain of unity, so zero dB
2/ power amp: anywhere between 26-29 dB

So overall gain of 26-29 dB.

With, let's say, 6 dB preamp gain from the dbx, that number would be 32 to 35 dB. 35 dB would be approximately the recommended system gain of THX.
 
Interesting observations.

Would you have an idea what is the overall system gain before you introduced the dbx?

My rough estimate with the B1 in the chain would be:

1/ B1: gain of unity, so zero dB
2/ power amp: anywhere between 26-29 dB

So overall gain of 26-29 dB.

With, let's say, 6 dB preamp gain from the dbx, that number would be 32 to 35 dB. 35 dB would be approximately the recommended system gain of THX.
I have no idea what the system gain was before I inserted the dbx. The thing that I'm puzzled is why my system requires so much gain before the final amplication to sound good. The thing is having a high gain and turning down the volume of the amp sounds better than low gain and turning up the volume of the amp. I have tried to keep the volume same as much as possible that my ear could decide. The low frequency extension, decay of the instruments and overall tonality sounds much better with high gain before final amplication.
 
@mbhangui when you wire balanced to unbalanced, there is straight 6db loss as pin 3 of balanced (-ve or cold as stated) output is disconnected on the umbalanced side.
You are using Pass B1 as input selector and another passive pre in the end before the amp as probably the master volume control. What type of attenuator is used in the final passive pre?
I would suggest you completely remove the volume control circuit from the Pass B1 and use it purely as input selector.
In your chain without the dbx, I think the combination of two passive pre (Pass B1 unity buffer and the the other preamp) could be the bottle neck.
Passive preams which usually use either a stepped attenuator or a potentiometer can constrain the bandwidth due to thier higher output impedance.
So possibly by boosting the input gain via dbx when using an unbalanced source or with the even higher gain of the Xonar balanced output, you are able to give more drive to your amplifier.
To check the actual boost, you can run a 1khz sine wave and measure the a/c output at the amplifier using a multimeter with and without the dbx at the same volume position on both your preamps.

Dbx is dping nothing other than boosting the signal voltage. So you need to check which passive pre is the constrain.
 
@mbhangui when you wire balanced to unbalanced, there is straight 6db loss as pin 3 of balanced (-ve or cold as stated) output is disconnected on the umbalanced side.
You are using Pass B1 as input selector and another passive pre in the end before the amp as probably the master volume control. What type of attenuator is used in the final passive pre?
I would suggest you completely remove the volume control circuit from the Pass B1 and use it purely as input selector.
In your chain without the dbx, I think the combination of two passive pre (Pass B1 unity buffer and the the other preamp) could be the bottle neck.
Passive preams which usually use either a stepped attenuator or a potentiometer can constrain the bandwidth due to thier higher output impedance.
So possibly by boosting the input gain via dbx when using an unbalanced source or with the even higher gain of the Xonar balanced output, you are able to give more drive to your amplifier.
To check the actual boost, you can run a 1khz sine wave and measure the a/c output at the amplifier using a multimeter with and without the dbx at the same volume position on both your preamps.

Dbx is dping nothing other than boosting the signal voltage. So you need to check which passive pre is the constrain.
Kannan,

The passive pre is just an output selector switch. Earlier it had a volume control, but i removed it. The output can be selected to go to 3 different amps. Now there is no volume control.

The pass b1 box has an input selector. The output of the input selector goes to a volume control (10 k) and the output of the volume control goes to the pass b1.

earlier the pass b1 output went to Rod Elliot's Preamp. The output of Rod Elliot's preamp directly went to the passive preamp (output selector switch). Also the Rod Elliot's preamp gain was at default setting of 9.93 dB

Now I inserted the graphic equalizer between the pass b1 and Rod Elliot's preamp and found it sounds good. Then later I removed the graphic equalizer, but adjusted the gain of the rod elliot's preamp to be 12.9 dB instead of 9.93 dB and I again find the performance to be good.

As of yesterday, I have dismantled both the pass b1 and the passive preamp.

Now I plan to have one box with input selector and the Rod Elliot's preamp. It will also have a 4 pole switch to bypass the preamp. At the flick of the switch the input gets connected to the output and the preamp input and output gets disconnected (though it remains powered on). I have finished wiring the 4 pole switch to bypass the preamp. This box will have no volume control.

Second box will have volume control and output selector switch. The output of the volume control will go to the relay board (here I'm using input selector board in the reverse). It will be passive except that I will be using relay board to select the output and led indicator to display the output selected.

IMG_20220501_093833049.jpeg
 
have finished wiring the 4 pole switch to bypass the pream
Try with and without the DPDT or 4PDT switches in the chain as these switches can add their own resistance/impedance. Not sure if it will impede in the bandwidth, but a better solution for long term will be to go with 4-pole relays.
 
Purchase the Audiolab 6000A Integrated Amplifier at a special offer price.
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