Designing A brand-new exotic pre-amplifier from Scratch

[Hari Iyer]

I always wanted a good pre-amplifier to pair with my 300Bs. There were many tested designs available but always had the DIY itch to design something of my own. Also wanted to incorporate the newly tested phono pre-amp (point to point) into a pcb based design for sturdier use too. All this design goals results in a brand new design of a pre-amp.

The pre-amp will have a Phono RIAA equalizer that will use the below design with some changes to suit my cartridge loading. Also the listed LSK489 is hard and expensive to procure and hence am settling with other options.

http://www.waynekirkwood.com/images/jpg/LSK489_Phono_Preamp.jpg

The line driver will use the Burr-brown chip DRV137 which has a balanced output and will again ccombine them using another unity gain Burr-brown chip INA137.

The final output stage will be DCB1 buffer pre-amp. The total gain of this pre-amp will by 2 (6dB) and should be good enough th drive my 300B easily.

Enough noise reducing capacitors and impedance matching resistors will be employed to prevent cable capacitance loading and noise.

I have also ordered some kits from ebay to complement the pre-amp which is obviously the power supply board using the LM317 / LM337 regulated +15/-15 volt power supply which is better than the 7815/7915 module.

Also ordered the relay based source selector which has 4 inputs with the required selector switch and LEDs all neatly mounted on a PCB for easy fixing in the front panel.

Will get a 1A torodial transformer made from 'Tomson Electronics' at Lamington Road. I have build transformers earlier with him and he does an excellent job on that one with close to Zero % tolerance.

Other things to buy are the RCA/ Volume pots and plenty of connectors as i want this pre-amp to be free of any non-professional build look. Am still considering some design improvements before finalizing the design and will tweak them before i give the pcb for making.

Expecting the project to get over in next 3 to 4 months.

Thanks for looking.

 

[Hari Iyer]

Update

I have ordered a DAC kit using the Burr Brown PCM1794 which i am planning to integrate with the same pre-amplifier. Have today also sourced all the remaining components - jfet, op-amps, transistors required for the build. Hope this project will see the light of the day sooner or later.:yahoo:

 

[Hari Iyer]

Update

I have ordered a DAC kit using the PCM1794 DAC which i am planning to integrate with the same pre-amplifier. Have today also sourced all the remaining components - jfet, op-amps, transistors required for the build. Hope this project will see the light of the day sooner or later.:yahoo:

Yet to give the phono pre-amp for art work and pcb making.

 

[Hari Iyer]

Updates:

Received the DAC kit last week.
Received the RCA input boards / connector.

JFET input phono RIAA pre-amp pcb awaited.
Alpha 20 line pre-amp pcb awaited

Components awaited - BJTs, JFETs, op-Amp - expected by tomorrow.
Transformer awaited - expected this weekend.
VU meter driver - yet to order.

Pre-amplifier case - modification required

Will take atleast a month or two to complete assembly.

Thanks for looking,

 

[Hari Iyer]

Another hetic weekend getting some parts / components from Lamington Road and again assembling the stuff.

Checked the +/- 15 volts class A power supply that will power the phono-preamp and DCB1 and its okay.

Also completed the DAC assembly and tested with my reference track. My GOD its too good and i have not heard anything like this before. The PCM1794 is very resolved with flat low, mids and highs. No frequencies are either over emphasised or laid back. The mid-range clutter just vanished and highs are more in line with the mids and lows without any sibilance or screech.

I may go for a op-amp swap later just to check out if the AD827 can be bettered by the LM4562 and will post my findings later.

Also connected my Dish TV to the optical input of the DAC to test other inputs and was too good to watch movies and music with this DAC with real life like performance.

There is a small hitch in the USB input being not able to read the input sample. Will try and find out why later and may be the usb version is not compatiable with my laptop.

All in all fully satisfied with the outcome of this exercise and can now say that my Digital processing is some what completed with this DAC kit.

Will complete the phono pre-amp and DCB1 next week. Till then keep watching.

Cheers,

 

[Hari Iyer]

The phono pre-amp + DCB1 pcb is now ready for printing and assembly may begin this weekend if i dont visit the What hifi show this weekend. Else will have to wait for another weekend.

 

[bhagwan]

The phono pre-amp + DCB1 pcb is now ready for printing and assembly may begin this weekend if i dont visit the What hifi show this weekend. Else will have to wait for another weekend.

Why ?
Do you not want to see & listen to all the audio gear ?
The Pre can be built after 1 week.
Else work @ night - the Show only happens once each year !
YOU MUST COME !

 

[Hari Iyer]

Why ?
Do you not want to see & listen to all the audio gear ?
The Pre can be built after 1 week.
Else work @ night - the Show only happens once each year !
YOU MUST COME !

Thanks. I am coming tomorrow early morning. Should be there around 11:30AM. Plan to spend 3 to 4 hours if possible (Depends upon the initial 1 hour impression of the show). Last year's show did not impress me much and none of the speakers displayed were up to the mark for the money quoted imo. Only speaker i could still remember listening to was the Epos floor stander. Hope to find some show saver this time around w.r.t. speakers.

 

[bhagwan]

Thanks. I am coming tomorrow early morning.
Last year's show did not impress me much and none of the speakers displayed were up to the mark for the money quoted imo.

Respected Sir,
It is worse this year.
The Speakers start @ 1 Lac & go to 75 Lac
Value is a rather personal thing.
If you want to come to the show to experience Hi Fi !
See
Touch
Listen
Etc.
Its a great place to experience imo.
However, None of the products are VFM.
So leave the price idea at home when you come.
Listen - Pure Performance in mind.
The Cadence Room has audio gear worth 1 Million [USD] in it - i.e. Rs. 7/- Cr.
Just come. See & try to listen & understand what that room is doing.
The JBL Synthesis Room - 50 + Lacs [nothing is visible] but experience the HT. These are things worth absorbing. Try to understand & put it in perspective.
Trust me - you can only benefit from Audio Shows like this.
The Nordost Room is 100 % Worth a Visit.
Spend at least 45mins there. Mr. Alex Brady has so much knowledge & experience - that he wants to share with you.
Take advantage of that.
Dont make fun of the products just because they are not affordable.
First try and understand what they are doing - idea / design / philosophy etc.

Give it a try Mr. H. I. you will only stand to benefit.
All the Best !
Wee you on Saturday;

 

[spirovious]

I enjoyed live show Today.I do watch some well recorded YouTube hifi videos and always want to experience in person. It happened once again today-
http://www.hifivision.com/av-lounge/65826-whathifi-show-mumbai-3-5-march-2017-a-4.html#post728985

 

[Hari Iyer]

Yes, strongly concur what is value is very personal and that's what audio is all about - be it the hardware, software, what is good SQ or what is bad SQ. Every person is unique so are the products. None of them or good or bad products - its only how you could connect or relate to them. In my case - i have always noticed that 'YOU can NEVER BUY MY SPEAKERS - ITS THE OTHER WAY ROUND - THE SPEAKERS SHOULD BUY YOU'.

 

[bhagwan]

'YOU can NEVER BUY MY SPEAKERS - ITS THE OTHER WAY ROUND - THE SPEAKERS SHOULD BUY YOU'.

100 % Correct.
I cannot agree with you more.

 

[shafic]

Yes, strongly concur what is value is very personal and that's what audio is all about - be it the hardware, software, what is good SQ or what is bad SQ. Every person is unique so are the products. None of them or good or bad products - its only how you could connect or relate to them. In my case - i have always noticed that 'YOU can NEVER BUY MY SPEAKERS - ITS THE OTHER WAY ROUND - THE SPEAKERS SHOULD BUY YOU'.

+1
Well said Hari!!

 

[Hari Iyer]

updates -

- Completed the phono RIAA stage. Minor noise and hum noticed. Still figuring out how to fix it. Can think of Transformer change from EI core to R core. Replacing the Rubycon capacitors to Keltron, re-wiring the phono pre-amp input stage properly. Other than this the phono stage sounds awesome (in comparison with the CA Azur - 619 phono stage).

-DCB1 completed and wired in the final output stage with the input having the volume controls. Again sounds great with full isolation between source and amplifier. I am using an smd type jfet MMB5557 n-channel jfet for this.

- Pre-assembled PCM1794 DAC kit - again sounds awesome with all functions working as intented. The usb inputs are having some issues and am getting it resolved with the vendor directly.

All in all the outcome of the effort was nice with all my requirements getting fulfilled. Now i have to sit back, relax and enjoy the music for some time.

:yahoo:

 

[Fantastic]

If you have used the circuit values in the link you put up on your first post for the RIAA , you will have inflated HF due to an error in the resistor value. R12 should be 34.09K and not 3409 ohms. So instead of 75uS you get 7.5uS.
Otherwise the component values are fine.
You could implement this with a NE5534 also. That has been done a long time ago. I did want to try it but never got round to doing it. No one has reported on it's performance. But it should certainly be very good.If you use a NE5534 for the second stage, an inverted configuration would be better. If you worry about the inverted signal , you can always reverse the speaker leads ! Better than adding another inverting stage !
You can also skip C2, 0.1uF . That's right at the input and can colour the sound easily apart from being a potential source for hum pick up. A very good quality polypropylene or PIO types will be huge . Most other types will degrade the sound.

 

[Hari Iyer]

If you have used the circuit values in the link you put up on your first post for the RIAA , you will have inflated HF due to an error in the resistor value. R12 should be 34.09K and not 3409 ohms. So instead of 75uS you get 7.5uS.

Infact i have kept 3409 unchanged and modified the 2200pf to 22kpf for getting correct output. It again is 75us.

You could implement this with a NE5534 also.
You can also skip C2, 0.1uF . That's right at the input and can colour the sound easily apart from being a potential source for hum pick up. A very good quality polypropylene or PIO types will be huge . Most other types will degrade the sound.

I have implemented with LM4562 which is good too. The 0.1uf used is a metalized polyproplene. In my next mod i will be changing many components to keep the input noise floor low.
one 47pf will be across the inverting and non-inverting pins. One 4700pf at the o/p stage after the 10uf.
Some input cartridge loading values to change with some ladder network to keep noise floor down. Also will plan to change the feedback in the op-amp from current 10 to 22 as the o/p level is a bit low and need to increase volume to 1 o'clock position to get decent level.

Am planning the mods shortly and expect it to be better than v1.0.

 

[Fantastic]

...........
Some input cartridge loading values to change with some ladder network to keep noise floor down. Also will plan to change the feedback in the op-amp from current 10 to 22 as the o/p level is a bit low and need to increase volume to 1 o'clock position to get decent level.

What ladder network ? Where ? Are you having a noise problem currently ?
Decreasing feedback to get more gain will also reduce the overload threshold at HF. Since the first stage has no equalisation it's HF threshold is lower than circuits with nfb equalisation.
Might be better to have a gain stage after the phono preamp and before the volume control rather than increase the first stage gain? But then again this depends on the output of the phono cartridge. Plenty of scope for experimenting !
With a +/- 15 V supply and a cartridge with 5mV nominal output and a gain of 22 in the first stage and will have about 19dB headroom at 20 KHz . With a gain of 30 you will have just over 16 dB margin at 20 Khz. Could try both. Need to ensure you don't have ultrasonic clipping caused by the very high levels of HF content if you have clicks and pops from the disc.
What was that about the 4700pF after the 10uF at the output stage ? I didn't see any 10uF at the output. Did you use a DC decoupling cap there ? Was the dc offset so high ? There will be some capacitive loading on the output due to the interconnect cable / plugs and sockets. At least 250 pF or more I'd guess. Opamps don't like capacitive loads. Maybe you should use a 100 ohm series resistor at the output as is done quite often.

 

[Hari Iyer]

What ladder network ? Where ? Are you having a noise problem currently ?
Decreasing feedback to get more gain will also reduce the overload threshold at HF. Since the first stage has no equalisation it's HF threshold is lower than circuits with nfb equalisation.
Might be better to have a gain stage after the phono preamp and before the volume control rather than increase the first stage gain? But then again this depends on the output of the phono cartridge. Plenty of scope for experimenting !
With a +/- 15 V supply and a cartridge with 5mV nominal output and a gain of 22 in the first stage and will have about 19dB headroom at 20 KHz . With a gain of 30 you will have just over 16 dB margin at 20 Khz. Could try both. Need to ensure you don't have ultrasonic clipping caused by the very high levels of HF content if you have clicks and pops from the disc.
What was that about the 4700pF after the 10uF at the output stage ? I didn't see any 10uF at the output. Did you use a DC decoupling cap there ? Was the dc offset so high ? There will be some capacitive loading on the output due to the interconnect cable / plugs and sockets. At least 250 pF or more I'd guess. Opamps don't like capacitive loads. Maybe you should use a 100 ohm series resistor at the output as is done quite often.

I have used a 10uf capacitor at the output stage after the op-amp (metalized polyproplene type) and after that a 220 series resistor and a 220k parallel resistor. I am planning to add the 4700pf capacitor parallel to the 220K resistor to improve HF response and reduce any RF/EMI noise at the o/p stage.

The current gain is around 36dB @ 1KHz which is adequate for most recordings. I would like to add only a 3dB to 6dB gain ie.increase from 36dB to may be around max 45dB .

Currently i have an issue with my transformer and it will need replacement. Hence i will need to use the 10uf capacitor till that time due to dc offset issue.

 

[Fantastic]

It's always a good thing to use a good coupling capacitor at the output provided it doesn't affect the sound noticeably. You never know if something you connect to it later on might have a problem even with small dc offsets. You could always take it off ( use a jumper/shorting plug parallel to it ) if all's well.
However I think you should skip adding a 4700pF cap at the output. Chance of RF coming through all that circuitry is quite remote. In all likely hood the power amp input itself has a capacitor in parallel with the input to stop RF ingress. Always good to minimise any capacitive load. It could take the 'airiness' out of HF.

 

[Hari Iyer]

I have successfully completed the version 2 of the phono pre-amp and find it very intimating. It has wide sound stage, open and uncluttered midrange, airiness, better details, mellower mids and highs. The gain adjusted to suit my power amp also works as intented with no over boost of the response.

All input noise associated to cartidge loading has now vanished completely. This was achieved by not having any parallel capacitor at the input stage. My cable capacitance at the input stage itself had a capacitance of around 140pf which was more than enough for this phono stage. Adding another capacitor 100pf or 200pf would have increased it significantly adding to the noise figure.

All in all a well concluded project. Now back to just enjoying the music.:clapping: