Adcom gfa-535ii input impedance

According to the schematic of the Adcom gfa-535ii the input impedance appears to be 1mohm. That seems like alot. Was interested in possible experimenting with lower input impedance and realizing I would have to increase the input cap value. Is there any danger in doing this?
 

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According to the schematic of the Adcom gfa-535ii the input impedance appears to be 1mohm. That seems like alot. Was interested in possible experimenting with lower input impedance and realizing I would have to increase the input cap value...

Specs say "100K".

A full study shows a 1Meg, a 1.5Meg, and a 100K all AC-parallel on the input; the source must drive 85K.

The 1uFd cap only has to drive the 1.5Meg and 100K, 93K.

This is a 1.7uFd bass-cut. Plenty low.

_WHY_ do you want to drop the input impedance?? Conventional interfacing works best when the load is MUCH-higher than the source. Even very-long lines (across the city) drive well with hi-Z loading.
 

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Oh, I saw the 1mohm. I thought the first resistor to ground determined the input impedance. I will defer to you though. I wanted to change it based on a hunch that it might open up the sound a little. I am using solid state preamp. I saw it as a possible "choke point" and maybe it was the cause of what I was hearing when I turned the amp up. Thanks for the circuit explanation.
 
Well I changed the 1mohm resistor that goes from input to ground to a 220k carbon and it sounds better. So it might be worth experimenting if you own one. I've noticed that input resistor that is too high can stifle the music and too low and it sounds ragged and thin.
 
Well I changed the 1mohm resistor that goes from input to ground to a 220k carbon and it sounds better. So it might be worth experimenting if you own one. I've noticed that input resistor that is too high can stifle the music and too low and it sounds ragged and thin.

Perhaps you mean 1M ohm resistor instead of 1mohm?

I have several 535II Adcoms, but I can tell you I will not be changing out any of the input resistors to lower values or to carbon composition resistors.
I have changed out the 1uF MKC caps to better caps, though.

Best of luck with your Adcom input resistor experiments.:)
 
Yes that's it, the 1M. I also changed the 1uf ero input cap for a 10uf tantalum. The 1uf made the amp sound a little unsure of itself IMO. Now the amp sounds more dynamic and fun. Other adcoms that I looked at do not have such a high value resistor at the input. What other mods have you done to your adcoms?
 
Effect of Doubling Input Impedance on Adcom 535II

Hello, this is three years later, but related: I have a 535II driven by an LDR passive volume control. I plan on adding another 535II in parallel (along with another midrange) to the passive output and getting the same sound, but 6db louder.
In order to keep the gain curve the same, could I double the input impedance to 200Kohm? Would anything bad happen like oscillation? Would I have to also change any other components? Thanks! Paul
 
This amp has a bipolar input, which can be sensitive to the DC input impedance.
Also the integrator feeds into this same node, and expects to see the 100k.
Because of this, I would be hesitant to change the 100k to 200k.

What is the value of the volume control? If it is 50k or less, there should be little change
from driving ~50k instead of ~100k. If the control is 100k or more, the taper would be
somewhat changed, but it would still work ok. You would just turn it up higher.
 
An op amp source has such a low Rout that any loads over 2k won't make a difference
(unless there are series coupling capacitors).

An LDR volume control, if unbuffered on its output, will (like any volume control) be loaded,
and affected by the impedance of the load.

Perhaps you could connect the amplifiers as you wish, and also add extra 100k loading resistors
on the LDRs that only drive one amp. That way all the LDRs will see the same value of load,
and will all still track the same.

Can you describe this system, using so many LDRs?
 
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It's 7.1, but the front 3 channels have active crossovers at 140 Hz, 2 power amps each, for a total of 11 channels. Audio is above the noise at full line-level volume until the LDRs which are just before the power amp inputs. Variable DC voltage for series and shunt LDRs is controlled by two ganged 6-watt 100K audio taper pots that I got surplus. One knob, 11 channels.
In practice I listen to and try to improve mostly right-front and left-front. The other channels are for my wife, an attempt to get buy-in, you know, increase the Waf. Dramas in 5.1, etc.