Using the power amp of an integrated amp

ulogon

Member
2019-07-05 6:16 pm
I want to use the power-amp section of my stereo integrated amplifier with a separate preamp.

Inside the integrated amplifier really are combined a stereo passive-preamp with a stereo power-amp.

The heart of the passive preamp is an ALPS Blue Velvet Stereo Motorized Potentiometer with its related IN/OUT/GND pins.

To the OUT pins (L & R) of the pot coming 2 shielded cables from RCA-socket inputs and from IN pins (L & R) of the pot start 2 shielded cables that go directly to the Power-amp stages (L & R) of the integrated-amp.

Now my question is the following:

Desoldering from the potentiometer the 2 shielded cables that go directly to the Power-amp stages and then soldering to their ends 2 RCA sockets (L & R) and then connect to those RCAs sockets the stereo signal coming from a different preamplifier should be a working way without issues?
 

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Desoldering from the potentiometer the 2 shielded cables that go directly to the Power-amp stages and then soldering to their ends 2 RCA sockets (L & R) and then connect to those RCAs sockets the stereo signal coming from a different preamplifier should be a working way without issues?


Yes, that sounds right - do you know the sensitivity of the power amp though?
 

ulogon

Member
2019-07-05 6:16 pm
Yes, that sounds right - do you know the sensitivity of the power amp though?

Thank you!
Nope, I do not know that sensitivity.

I would add a 100k resistor across each RCA input, just to be sure there's a DC path to ground for the amp input after the control is disconnected. Probably already is, but.

Please note that the impedance of existing potentiometer is 10KΩ.
So in your opinion a 10KΩ resistor could be enough?
It has to be soldered just as in the pic in attachment?
Thanks!
 

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ulogon

Member
2019-07-05 6:16 pm
Yes, that sounds right

I would add a 100k resistor across each RCA input, just to be sure there's a DC path to ground
for the amp input after the control is disconnected.

Following your advices I connected a separate preamp to the power stage of my integrated amp and it works like a charm: thank you both! :up: :up:

rayma,
Please note that I added a 50kΩ resistor to RCA socket, but since the preamp output impedance is just 1Ω I think that also a lower value should work: do you agree?
Furthermore I'm just wondering how it would sound (related to its musicality) without that resistance, but I'm afraid to take it off because I don't know what could happen...
So, please what could risks be?
What did you exactly mean with "after the control is disconnected"?
Thanks!
 

rayma

Member
2011-04-29 8:37 pm
I added a 50kΩ resistor to RCA socket, but since the preamp output impedance is just 1Ω I think that also a lower value should work: do you agree?
Furthermore I'm just wondering how it would sound (related to its musicality) without that resistance, but I'm afraid to take it off because I don't know what could happen...
So, please what could risks be?
What did you exactly mean with "after the control is disconnected"?

Can you post the brand and model of your amplifier? It may be possible to tell whether
there is already a resistor inside. Any value 10k - 100k should work if there isn't.

There could be a preamp output capacitor that would roll off the bass with a too small resistor.
If there is no resistor inside, it's possible the circuit could be perturbed without one in place
of the volume control.
 

ulogon

Member
2019-07-05 6:16 pm
There could be a preamp output capacitor that would roll off the bass with a too small resistor.

Yep, after your addressing I've learned about that possible issue

Mix and match impedance | PS Audio

However I can confirm it does not seem that case since the sound is good.

just to be sure there's a DC path to ground

As said really I was afraid about the above statement since I'm not an expert...
Thanks!
 

Everyone's buddy Paul ... great guy, but a source of considerable ongoing disinformation about audio and hifi. I catch him out all the time.

Modern amps no longer require precise impedance matching. They use a system called Impedance Bridging that guarantees you can freely connect one device with another.

The source device (pre-amp, tape, CD, etc) will have a low output impedance, 75 ohms is typical. The input of the next device (pre-amp, power amp, etc) will have a high input impedance, 10k ohms is typical. In this way you can be sure that one device can feed another without degradation of the signal.

The signal levels are also standardized. One for professional audio (PLL) and one for consumer audio (CLL) as explained HERE

So it is pretty much mix and match, although there is no gurantee this is continued inside each device.
 
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ulogon

Member
2019-07-05 6:16 pm
What brand and model is your integrated amplifier?

AMAudio T-90

Features translation (if useful in some way) in attachment.

However during my trials I've replaced the 51k resistor (> good sound) with a 7.5k one and the sound got worse.
Then I've replaced the 7.5k with 2x51k resistor (102k) and the sound is good again, nay improved.


Everyone's buddy Paul ... great guy, but a source of considerable ongoing disinformation about audio and hifi. I catch him out all the time.

Modern amps no longer require precise impedance matching. They use a system called Impedance Bridging that guarantees you can freely connect one device with another.

The source device (pre-amp, tape, CD, etc) will have a low output impedance, 75 ohms is typical. The input of the next device (pre-amp, power amp, etc) will have a high input impedance, 10k ohms is typical. In this way you can be sure that one device can feed another without degradation of the signal.

The signal levels are also standardized. One for professional audio (PLL) and one for consumer audio (CLL) as explained HERE

So it is pretty much mix and match, although there is no gurantee this is continued inside each device.

Thanks for your appreciated reply, I'm just learning more and more, but please note that the topic is related to a power amp stage of an integrated amplifier and I do not know if that is just matter.
 

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