Volume controls, impedance, Gain & help :-)

[tonescout]

Hi,

I am trying help a friend cut down the gain of an old Valve Power amp and a very high gain valve pre amp. He does not want to change anything in the amplifiers if he can help it. Through experience using his preamplifier feeding an integrated - so effectively balancing his valve preamp gain with the integrated gain he found a good balance.

I said I would help making him a voltage divider / single step attenuator so that he can have the equivalent to a passive stepped attenuator at a single step to drop down the output of the valve preamp to the valve power amp in his system. This will sit in between the Valve pre and the valve power amplifiers.

His valve pre amp has an output impedance of 1k, and his power amp has input of 100k. So I was thinking making the RT of the voltage divider 10-20K?

Thoughts welcome

 

[artosalo]

Put 50k potentiometer between those amplifiers and adjust for desired gain/sensitivity. Then measure potentiometer halves and replace it with resistors.

 

[tonescout]

Thanks - good idea. Why did you choose 50k, not say 20k?

 

[artosalo]

Any value, say 20k...500k is OK.

 

[tonescout]

ahh - ok, I just read a post around choosing the right impedance match and thinking this was important? I note the manual of the preamp suggests 20k and the other post was talking about a 10x for each output/input?

 

[audiowize]

Just be sure that you build this into the input of the power amp.

 

[tonescout]

Thanks - I was going to build a little box and have short captive leads into the power amp.

Any more ideas on the overall RT for the divider? I am still thinking about 20k?

 

[audiowize]

Can you say specifically what the preamp is?

If there's a 12AX7 cathode follower at the output, I'd lean more on the 20+k side of things.

 

[tonescout]

It's the audio research SP10 (26 dB gain....74 on phono) and D125 (110W).....driving Klipsch Heresy speakers at 99 db/W

SP10 is all 6DJ8 https://www.arcdb.ws/Database/SP10/ARC_SP10_manual.pdf
D125 here ARCDB

Thanks!

 

[audiowize]

Yeah, that's a classic case of way too much gain and speakers that are far too sensitive.

You have 26dB of gain in the preamp and 32dB of gain in the amp.

Let's say you have a DAC with 2V of output and your speakers can use a peak of 10W, that means 13dB of gain between your source and speakers is sufficient. I would look for a small integrated amp with ~15dB of gain, or you could put a 45dB pad between amp and preamp.

The AR preamp should cope just fine with a 10K load.

 

[PRR]

There's a -12dB low-gain switch on the SP10, FWIW.

45dB pad seems extreme, but I can certainly see 20dB loss between preamp and power amp; trial/error may be the best guide.

(A 15k:8 transformer off the SP10 output *may* drive a speaker well, leaving the D125 unemployed.)

 

[audiowize]

No, a 15K:8 transformer is not going to drive a speaker well. The output stage of the SP-10 runs 10mA, which is great for a preamp but nowhere near enough to drive a speaker, even with the step-down transformer.

 

[PRR]

Thank you for your opinion.

 

[audiowize]

Yes, my opinion is that ~700mW isn't enough to drive a pair of Klipsch Heresy speakers.

 

[stocktrader200]

Use 33k into 7.5k 40k load to preamp
About 1/4 output to amp

 

[tonescout]

Thanks for this.

So the SP10 at 1K sees 40K at the volume attenuator which then sees 100k at the power amp.

I found this site which is useful too 50-Step Shunt Attenuator Resistor Calculator - Neville Roberts

So that is nearly 15 dB cut.

Why did you choose 40k at RT, that would be useful to understand?

 

[painted]

With pre Zout=1k and 40k to ground the circuit interface is fine (ca 2,5% voltage loss). Next you have to consider the input impedance at 20KHz of the connecting stage, which is (with 1k+7,5k and some added parasitic capacitance) also fine.