Reduce the gain on a SET 120 amp?

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Hear Clifford Brown

Reduce the gain on a SET 120 amp?
« on: 10 Feb 2024, 04:10 pm »
I have a SET 120 (not the control amp) and can only go up 4 or 5 clicks on my passive volume control before it gets very loud.  I'd like to replace the passive with a tube preamp but that will add gain, which I assume would mean a very limited useful range of the volume control.  I want to continue using the balanced output of my DAC which is 4 volts.  Speakers are 8 ohm and very efficient around 95dB.

Does anyone know if the gain on the SET 120 can be lowered?

Tone Depth

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Re: Reduce the gain on a SET 120 amp?
« Reply #1 on: 10 Feb 2024, 04:20 pm »
You can either hardwire in higher resistance in your amp, or use the attenuator in your preamp to increase the resistance in that circuit, to decrease the signal level going to your amp.

Hear Clifford Brown

Re: Reduce the gain on a SET 120 amp?
« Reply #2 on: 10 Feb 2024, 04:31 pm »
Thanks for the reply Tone Depth. I don't understand the second suggestion which might mean I have a fundamental misunderstanding of what the volume control on a preamp does.  Lets say it has a stepped attenuater with 0.5dB steps.  With the control all the way to the left is max attenuation. Step one 0.5dB less attenuation, step two 1.0dB less attenuation etc.  Is that correct?  If step three is already too loud, how do you use the attenuator to increase resistance in the circuit?

danielgk

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Re: Reduce the gain on a SET 120 amp?
« Reply #3 on: 10 Feb 2024, 09:45 pm »
Reduced gain can be done.  Problem is by how much?  4v out of the DAC is obscenely high and 95db speakers are obscenely efficient.  If you modified a SET120 to be perfect for this system it would mean cutting the gain by about 4x (from about 23 down to around 6).  This would make it fairly useless in many other systems. 

I suspect you might be right in that swapping the passive volume control for a preamp with gain and a volume control could make matters worse, but I don't think there's any way to tell without trying it.  I can see it going either way, or staying the same.  It would all depend on the preamp design. 

You could always go DAC to tube preamp (with volume control) to passive stepped attenuator to SET120.  Or DAC to passive stepped attenuator to tube preamp (with volume control) to SET120.  (Just DON'T put the stepped attenuator between the power amp and the speakers).


Dan

Hear Clifford Brown

Re: Reduce the gain on a SET 120 amp?
« Reply #4 on: 10 Feb 2024, 10:13 pm »
Great answer, thanks Dan.  Encouraging to hear you say "I can see it going either way, or staying the same."  I could live with a tube preamp that gave the same usable volume control range I currently have with DAC > Passive vol control > Amp.

Also the option you suggest: DAC to tube preamp (with volume control) to passive stepped attenuator to SET120 sounds like a real possibility to me.

I have heard that today's DACs with balanced architecture sound best using their balanced outputs.  I think the first step here for me is to try the single ended out from the DAC and see how I like it.  I have a Denafrips Pontus and this is from the spec sheet:

Analog Output
RCA at 2.0Vrms, 625 Ω
XLR at 4.0Vrms, 1250 Ω

GeorgeAb

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Re: Reduce the gain on a SET 120 amp?
« Reply #5 on: 11 Feb 2024, 01:19 am »
You could insert a two resistors between preamp and amp and take output from between two resistors which is a voltage divider.  You need to be mindful of the input impedance as you want to maintain a typical input impedance of 47K, so your preamp does not have to work too hard (draw more current then designed to). For example a typical input impedance of an amplifer is 47K. If you used two 30K resistors and picked voltage between them to the input of the amplifer, you would maintain a 47K ohm load, actually 48K ohms, but close enough. The input going into the amplifer would be halved with this example. Once you got it dialed in, you could put the two resistors (divider network) inside the preamp, just inside before the rca output jack or in amp just after the rca input.

If you don't want if halved, figure out how much attentuation you want and choose resistors that provide output you want (Vout = Vin * (R1/(R1 + R2)) while maintaining 47K input Z (Z = R1+(1/(1/R2+1/47K)). Assuming Set 120 has input Z of 47K; no standard, but compentant designers use around 47K. Just a little math. First resistor is in series and added to he parrallel combination of second resistor and input Z of amp.

Just looked a pic of Set 120. Plenty of room to put the two resistors. Totally reverasable if you ever sell. 

Per this post the input Z of the Set series is 60K ohms, so you would want to use 60K for your calculations. With the example of the two 30K resistors the input would be 50K ohms. https://www.audiocircle.com/index.php?topic=159180.40




« Last Edit: 11 Feb 2024, 04:20 am by GeorgeAb »

Hear Clifford Brown

Re: Reduce the gain on a SET 120 amp?
« Reply #6 on: 11 Feb 2024, 05:09 pm »
Thanks George, I have a friend who works in electronics who could help put that together.  Looks like a clean simple solution.

GeorgeAb

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Re: Reduce the gain on a SET 120 amp?
« Reply #7 on: 11 Feb 2024, 06:53 pm »
Sounded (luv puns) like you were at bottom of volume knob and wanted to go to mid way or 1/3 of way clockwise.

One more pic to show how straight forward the mod is. Using standards resistors are values, closest is 33K. Also some examples of different combinations to reduce input as a percentage. Also 1/4W or 1/8W Vishay resistors are fine. Likely $.50 each, biggest expense will be shipping. Recommend orderning from Mouser, Digikey and not Amazon so you know you are getting a quality resistory like Vishay.

33K, 33K, = 50% volume reduction, input Z = 54K
27K, 33K, (top, bottom resistor) = 45% volume reduction, input Z = 48K
22K, 33K, = 40% volume reduction, input Z = 43K

If you need reduction more like just 30% then you need to increase value of second resistor to keep input Z around 47K.

Lastly if you know the percentage you want to reduce input by, let me know and will provide resistor values that will accomadate while keeping acceptable input Z.




GeorgeAb

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Re: Reduce the gain on a SET 120 amp?
« Reply #8 on: 11 Feb 2024, 09:08 pm »
Two more things and I will let this go.

When soldering to the female RCA always insert a male RCA into first. This allows heat to flow to other metal pin so it doesn't melt insulation in female RCA.

These jumpers work well to set up your divider network between two interconnects and test everything out before you start modifying equipment.
 
https://www.amazon.com/Crocodile-Alligator-Electrical-Insulators-Connection/dp/B09RZQFGGH/

Hear Clifford Brown

Re: Reduce the gain on a SET 120 amp?
« Reply #9 on: 11 Feb 2024, 10:15 pm »
Okay now it's clear.  The resistors go inside the amp.  And thanks for the listing of resistor values and the associated volume reduction.


GeorgeAb

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Re: Reduce the gain on a SET 120 amp?
« Reply #10 on: 11 Feb 2024, 11:35 pm »
I was going to leave this alone, but found an error in my calculations when calculating the % of reduction. :duh: 2nd resistor is in parrallel with 60K Z of of amp. So to determine % amp sees = 60K " R2/ (R1 + 60K " R2), where R1 top resistor and R2 is bottom in parrallel (")with amp impedance. All good now.

These are corrected:

R1, R,2
33K, 33K, = 60% volume reduction, input Z = 54K
27K, 33K, (top, bottom resistor) = 56% volume reduction, input Z = 48K
22K, 33K, = 50% volume reduction, input Z = 43K

For less reduction:
20K, 60K, = 40%, volume reduction, Z 50K ohms
10K, 60K, = 25%, volume reduction, Z 40K ohms
10K, 100K, = 21% volume reduction, Z 47.5K ohms

The idea of the jumpers is you put the divider between the two interconnects using jumpers to make connections. Make sure everything is working to your liking (volume knob is where you want it) before you start unsoldering and soldering. 

RDavidson

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Re: Reduce the gain on a SET 120 amp?
« Reply #11 on: 12 Feb 2024, 01:03 am »
I’m curious why you’re using the balanced out from the DAC. 4V is excessive gain to begin with. Unless your system is balanced from source to amp, there’s zero benefit to using the balanced connection in only one link in the chain. So, if you can run single ended from your DAC, that’s 50% of your gain gone immediately without breaking out the soldering iron and resistors.

Hear Clifford Brown

Re: Reduce the gain on a SET 120 amp?
« Reply #12 on: 12 Feb 2024, 04:43 am »
I used the single ended outs from the DAC last night and surprised me there wasn't much difference in gain.  Instead of 5 clicks up on the attenuator with balanced, it was 8 or 9 with single ended to get to the same loud output.

Not sure if I heard any difference in sound quality, I would want to compare RCA and XLR some more before concluding.  Suffice it to say there is not much difference.

The Pontus DAC is an R2R balanced DAC and in WaveTheory's review at around 28 minutes in, he seems to be saying it is possible depending on the internal architecture, that the RCA outputs only use half the R2R ladder meaning the RCA outs only use half the DAC.  Granted he could be wrong, but based on what he says here and what I've heard in other reviews, my conclusion was best to use the XLR outs.

https://www.youtube.com/watch?v=L4EvvhFjjYk&t=1778s