Speaker level to line level attenuator

[Subbu68]

When searching for a sub-woofer to augment Heresy IIIs in 2019 wanted it to be with Speaker Level inputs. The tube amp has only speaker out. Not keen to use a Y adaptor at it's input to feed subs with line level inputs only. Sealed sub was another aim. Primarily to match the fast Klipsch Heresy IIIs.

Only REL, KEF and SVS SB1000 seemed to tick the boxes. Finally went in for KEF Kube12b that was locally available. It has also an EQ for placement. So much for the choice of the sub.

On connecting the sub, was in for a nasty surprise. The sub was cutting off when volume was low or if the song did not have enough "thump". Was thinking of getting a commercially available line level attenuator to bring down the signal to line level and feed to the Line Level inputs of the sub but set on making it.

The sub had been working well since then. Sharing for those who have similar issues.

The above is for one channel. Resistors are 1 & 2W, 5%. I built on a vero board.

Till yesterday it was housed in a small jewelry box with a screw connection to the wires from amp and two RCAs for the output to sub.

With ACAs joining the camp, have two amps to contest for a share of the lonely sub. Set the same in a proper project box with a 5pin socket for input.

Yes, I have to manually do it - unplug Elekit Amp and plug in ACA and vice versa. Still, the two can share the sub without resorting to a screw driver to change wires.

 

[rajan-]

@Subbu68

I just got a REL T9/i , as it is on clearance. (T9/x replaced T9/i). Got a really good deal that I couldn't pass..

and I was reading on how to connect a REL high level connection to the Monoblocks.

REL does not recommend a single Sub to a differential monoblocks. But I do not want to get 2 Subs.

I have few theories to try based on this article : https://relsupport.zendesk.com/hc/en-us/articles/227195327-Balanced-Differential-Monoblocks-Part-I


Have you connected the single sub to a pair of Bridged ACA ?
have you used high level connections ? or low level from Pre-amp ?

I am curious to know the experience ..

As the speaker out polarity changes when you use the ACA in "Mono XLR bridged" mode and "Mono RCA Bridged" mode, tricky to take the high level connection to a line level attenuator

Details are here: https://guides.diyaudio.com/Guide/Amp+Camp+Amp+V1.8+Change+Information/10

 

[Subbu68]

@Subbu68

I just got a REL T9/i , as it is on clearance. (T9/x replaced T9/i). Got a really good deal that I couldn't pass..

and I was reading on how to connect a REL high level connection to the Monoblocks.

REL does not recommend a single Sub to a differential monoblocks. But I do not want to get 2 Subs.

I have few theories to try based on this article : https://relsupport.zendesk.com/hc/en-us/articles/227195327-Balanced-Differential-Monoblocks-Part-I


Have you connected the single sub to a pair of Bridged ACA ?
have you used high level connections ? or low level from Pre-amp ?

I am curious to know the experience ..

As the speaker out polarity changes when you use the ACA in "Mono XLR bridged" mode and "Mono RCA Bridged" mode.

Details are here: https://guides.diyaudio.com/Guide/Amp+Camp+Amp+V1.8+Change+Information/10

Hi Rajan

I did connect the sub through this attenuator to ACA bridged monos speaker outs. I got a crosstalk and it is to be expected as the right channel feeds the left in the ACA in bridged mono and the speaker -ve of ACA goes to common ground of the sub at the rca. I have to analyse the circuit and find a solution.

Yes, I think while writing this I got the solution. To invert connection to the attenuator? At ACA the DC negative bus becomes the +ve for speaker isn't it? And then maybe to invert phase with the phase switch of the sub?

Now I am away from my system for a few days to try out.

I did not use the low level signal since I read high level is the best to keep the timing.

On my Elekit no issues at all as speaker out are regular ones. This attenuator works like a charm.

I did not use the inbuilt high level inputs of the sub since it's auto cut off sensed no signal at low volumes and put the sub to sleep.

 

[greenhorn]

If your amp has bridged speaker outputs then connecting the negatives to a common ground is going to be a recipe for disaster

 

[Subbu68]

If your amp has bridged speaker outputs then connecting the negatives to a common ground is going to be a recipe for disaster

My bad I did not think of it while connecting the attenuator to the amp terminals. I just followed Red and Black wires of main speakers. One day had to switch off one of the two amps and lo!! The other speaker was singing . Then it clicked that something was wrong and disconnected the attenuator to find it normalised.

Now the sub is not used with ACA. And I am at my home away from home. When I get back, first thing to do.

 

[rajan-]

Yes, I think while writing this I got the solution. To invert connection to the attenuator? At ACA the DC negative bus becomes the +ve for speaker isn't it? And then maybe to invert phase with the phase switch of the sub?

AND the speaker connection has to change if you change from Bridged Mono to Balanced mono.

Keen to know what you come up with..

REL recommendations for Monoblocs says to leave the black (earth) wire floating...

 

[Hari Iyer]

For my subwoofer plate amplifier i am using a simple voltage divider network, nothing fancy, a single 10k and 3.3kin L-pad configuration and i am happy. This gives me around -10dB attenuation

 

[Subbu68]

For my subwoofer plate amplifier i am using a simple voltage divider network, nothing fancy, a single 10k and 3.3kin L-pad configuration and i am happy. This gives me around -10dB attenuation

It's the same here. The circuit is in the first post.

I added a couple of switches and another two resistors to have additional range.

 

[rajan-]

My bad I did not think of it while connecting the attenuator to the amp terminals. I just followed Red and Black wires of main speakers. One day had to switch off one of the two amps and lo!! The other speaker was singing . Then it clicked that something was wrong and disconnected the attenuator to find it normalised.

Now the sub is not used with ACA. And I am at my home away from home. When I get back, first thing to do.

I just connected my new REL Sub using high level connections to bridged ACA.

working very well.

I did not follow the speaker cables, rather looked at the ACA connection diagram to identify the high level outputs for each channel.

to be on the safe side, I left the REL ground wire floating. and I do not have any humming.

Note, I have done this only for the bridged mono connection.
Have not tried the balanced mono as I do not have a balanced mono pre-amp to feed the ACA

 

[Subbu68]

I just connected my new REL Sub using high level connections to bridged ACA.

working very well.

I did not follow the speaker cables, rather looked at the ACA connection diagram to identify the high level outputs for each channel.

to be on the safe side, I left the REL ground wire floating. and I do not have any humming.

Note, I have done this only for the bridged mono connection.
Have not tried the balanced mono as I do not have a balanced mono pre-amp to feed the ACA

Would try it out hopefully next week.

REL have a three pole input. Is it just that they have a common ground for two channels?

In KEF Kube either it is RCA or an inbuilt attenuator with a seemingly common ground but brought out separately for the two channel connection. Not sure how it would get a reference of i left the ground of the sub floating. If the LF of the two channels are in phase it'd cancel at the input perhaps resulting in no sound?

If I connected the actual negative poles of the ACAs at the sub maybe it's going to set up a ground Loop since they are common at the pre-amp out.

 

[rajan-]

REL has a speakon connectior for the high level inputs, its 3 wires to connect to amp on one side and speakon connector at Sub side

Yellow/red goes to each speaker and black common earth.

REL also has RCA Low level inputs and separate LFE inputs.

REL recommends to leave the black floating while connecting to balanced or differential monoblocks, and if hum happens an easy way out is to use a RCA wire to connect LFE input to An preamp connection. Th RCA collar will give common ground.

How To Connect A REL to a Balanced AMP | REL Acoustics

For differential (i.e. fully balanced) amplifiers using one REL, simply use the standard connecting scheme with the exception of NOT connecting the black

rel.net

Amp Camp Amp - ACA

Hello So what i have been trying is to get a youtube video with mono vs stereo and speaker test that swaps left/right with same sound. It's clear that in every case one channel is significant better than other, if i swap the rca between B1 and ACA the also swaps. I have tested 3 different rca...

www.diyaudio.com

and validated these info with ACA 1.8 connection diagram

Amp Camp Amp V1.8 Change Information

ACA V1.8

guides.diyaudio.com

 

[rajan-]

@Subbu68 I am planning to do some tests using low level input on the sub, fed by the pre-amp outs and compare it with the high level connections from the bridged power amps.

Unbalanced Stereo Line Audio Isolation Transformer 2 RCA - 2 RCA Isolator A2517

This unit is designed as an isolation unit to prevent hum loops in audio systems.

www.selby.com.au

split RCA out of the preamp --> RCA Isolation transformer --> use RCA Y cable to combine both channels --> Sub Low level input

Some of the comments and suggestions online is saying to use a y cable to combine 2 channels, which I dont want to do for obvious reasons. thats why the isolation transformer in the middle.

other choice was a mini DSP and summing stereo for sub out. I am trying to stay out of DSP for now...

thoughts ?

 

[Subbu68]

@Subbu68 I am planning to do some tests using low level input on the sub, fed by the pre-amp outs and compare it with the high level connections from the bridged power amps.

Unbalanced Stereo Line Audio Isolation Transformer 2 RCA - 2 RCA Isolator A2517

This unit is designed as an isolation unit to prevent hum loops in audio systems.

www.selby.com.au

split RCA out of the preamp --> RCA Isolation transformer --> use RCA Y cable to combine both channels --> Sub Low level input

Some of the comments and suggestions online is saying to use a y cable to combine 2 channels, which I dont want to do for obvious reasons. thats why the isolation transformer in the middle.

other choice was a mini DSP and summing stereo for sub out. I am trying to stay out of DSP for now...

thoughts ?

My view of not using preamp out was once the volume of sub is set with the main speakers, the amp volume controls the final output when using the Elekit amp.

Now with preamp controlling the volume, it may not be an issue. But at certain volume level will the sub overpower the main?

Somewhere i also read the timing of sub matches with the main speaker with high level input. Yes, phase will be same but will the plate amp introduce a delay due to electronics in it and sub will respond after a miniscule time delay T?

So that way is it better to use preamp out to sub RCA?

 

[rajan-]

My view of not using preamp out was once the volume of sub is set with the main speakers, the amp volume controls the final output when using the Elekit amp.

Now with preamp controlling the volume, it may not be an issue. But at certain volume level will the sub overpower the main?

Somewhere i also read the timing of sub matches with the main speaker with high level input. Yes, phase will be same but will the plate amp introduce a delay due to electronics in it and sub will respond after a miniscule time delay T?

So that way is it better to use preamp out to sub RCA?

any Powered Sub will have a Time delay.

Thats one of the reason for suggestions on feeding the sub first and taking the speaker outs from the subs to the front left and right.
then use the DSP algorithms to manage the TA. Bass augmentation setups like this is very common. checkout any good pro or studio sub, this kind of setup is common. Companies like Dynaudio (and many consumer brands as well) even has custom matched their own line of speaker to sub profiles built in to the DSP setup choices to make integration easier.


I read some where that the modern REL has 7 msec delay (compared to over 25msec on older models)

Need to experiment a little more. with bridged / monoblock to a single sub, I think I had to crank the sub volume a bit higher than when connected to a single power amp. not sure if thats due to each line being out o phase with each other, and at this point I dont want to experiment on wiring changes and blow the sub or the worse the amps.

thats why I am thinking of trying out the low level connection (but need to innovate some as my REL only has single RCA input. The higher end RELs have dual RCA lw level connections, so you wouldnt need to do the mono summing )

if this experiment does not give me the intended results and if I think high level connection (on a single AMP) is better, then I would feed the REL with a dedicated amp. There are heaps of used amps available on FB marketplace. At this point I cant go back to a single amp feeding the Main full rangers. just not good as bridged monoblocks feeding them.

 

[Subbu68]

@rajan-
Yes, back to my house in Abu Dhabi and my audio system. Tried rewiring REL way for the sub and reversing the input to the attenuator from the conventional speaker +ve as i used earlier to as per ACA circuit.

Bad hum without earth connection. Ran a wire from the preamp phone ground terminal to the sub RCA sleeve. It is now clean. No cross talk between the channels.

As i wrote in the intro of the OP, the direct high level input was not working with the Elekit amp as voltage was not high to keep it on always. Auto standby was switched in often. so had to go for the attenuator and input to the RCA of the sub.

 

[rajan-]

I gave up trying to experiment with
Isolation transformers
and Y cable
and Resistor Stereo to Mono summing circuits explained in http://silentsky.net/wordpress/archives/624
and some more...

with a single channel input to REL 's Low level input for a few days.

when all else is a no go, turn to professionals. in my case, pro gear.

finally, got a very economical solution from

Behringer | Product | MX400

www.behringer.com

This is cute and economical mixer take 4 channels and mix it to single Mono. Perfect for taking dual stereo out of Preamp and using some RCA to TS adapters (very common to electric guitar players) fed the signal to MX400 .

no more cross talk, humming, nothing... no issues... clean sound so far.

Just need to better integrate the level (on MX400, gain on the Sub and cross over on the Sub.

Back to back comparison with High level and low level sub inputs--- after its settled to my old and my son's young years.

dis-advantage is, some more knobs to play with and more mucking around. lol