Adcom GTP-400 too much gain!

[so_ein_pech]

Does anyone else find that their GTP-400 has way too much sensitivity in the volume control. I can't even turn it more than a quarter of the way up before it becomes stupidly loud.

Normally, I don't care too much about a sensitive volume knob but I am having some problems with this one. When I turn it just a few degrees above zero, the balance is off (i.e. one channel is much louder). Turn it a little more, and the channels are balanced again--but now they are too loud for quiet listening.

The amp I am using is an Adcom GFA 5300, which is really not that powerful, so it shouldn't be behaving like this. The GTP-400 manual lists the maximum output voltage at >10V so maybe it was designed to be this sensitive.

I think you have to be crazy want your amplifier to ever see that many volts. For reference the GFA has a fixed gain of 29dB so 10V input = ~280V at the speaker terminals.

math for those that care:
dB = 20*log(Vout/Vin)
db/20 = log(Vout) - log(Vin)
Vout = Vin*10^(dB/20) = 10*10^(29/20) = 281.8

I'm thinking of sticking about sticking a resistor in series with the volume control (at the input of the voltage divider network). The VR is 50K. Any thoughts about values to try? Or other ways to reduce the gain. Maybe a line-level signal attenuator?

 

[ryuuoh]

If you can find a compatible pot, you could go for a 100~250k audio taper pot.

 

[mhedges]

Does anyone else find that their GTP-400 has way too much sensitivity in the volume control. I can't even turn it more than a quarter of the way up before it becomes stupidly loud.

Normally, I don't care too much about a sensitive volume knob but I am having some problems with this one. When I turn it just a few degrees above zero, the balance is off (i.e. one channel is much louder). Turn it a little more, and the channels are balanced again--but now they are too loud for quiet listening.

The amp I am using is an Adcom GFA 5300, which is really not that powerful, so it shouldn't be behaving like this. The GTP-400 manual lists the maximum output voltage at >10V so maybe it was designed to be this sensitive.

I think you have to be crazy want your amplifier to ever see that many volts. For reference the GFA has a fixed gain of 29dB so 10V input = ~280V at the speaker terminals.

math for those that care:
dB = 20*log(Vout/Vin)
db/20 = log(Vout) - log(Vin)
Vout = Vin*10^(dB/20) = 10*10^(29/20) = 281.8

I'm thinking of sticking about sticking a resistor in series with the volume control (at the input of the voltage divider network). The VR is 50K. Any thoughts about values to try? Or other ways to reduce the gain. Maybe a line-level signal attenuator?

I use the Harrison Labs 12 dB attenuators to tame gain issues. Works well.

 

[whoaru99]

I think you have to be crazy want your amplifier to ever see that many volts. For reference the GFA has a fixed gain of 29dB so 10V input = ~280V at the speaker terminals.

There is no way in this life that you're going to get 280V at the speaker terminals of that amp.

Divide 280V by 12 and you'd about be in the ballpark.

 

[Dr Tinear]

There is no way in this life that you're going to get 280V at the speaker terminals of that amp.

Divide 280V by 12 and you'd about be in the ballpark.

Quite true. The power rating of 80 WPC into 8 ohms suggests that the amp will deliver just over 25 V RMS. The input voltage required for that power output is 0.9 V. My recommendation would be to build or buy passive attenuators and install them between the preamp and the amp. 12 dB, as mhedges suggested, would be about right. Installing them between the amp and preamp will reduce the line stage's noise along with the signal, giving the OP the reduced gain he wants without impairing the system's signal-to-noise ratio.

 

[so_ein_pech]

There is no way in this life that you're going to get 280V at the speaker terminals of that amp.

Divide 280V by 12 and you'd about be in the ballpark.

Well, yeah but you are missing my point. I was trying to illustrate why 10V of output from a preamp is excessive.

 

[whoaru99]

You'd need to have a relatively strong input to get 10V out, considering the gain of the pre seems to be about 16dB.

That said, the Forte F44 preamp I occasionally use acts the same way...barely have to turn the volume to get big change in output. Fortunately, the amp I usually use it with has gain controls so I just dial those back and all is good.

 

[so_ein_pech]

Reviving this thread because I finally got around to dusting off my GTP-400 and hooking it up to an oscilloscope. I am now thinking that the volume pot installed in it is simply bad. I can see that there no gain on the last 3rd of the dial. Even with a very low input signal it acts the same, so that I know its not some internal protection circuitry limiting the the output. Perhaps they accidentally used a linear pot or even a reverse audio taper pot when they built this thing.

Im thinking that a Bourns PDB182-K430K-503A should be a drop-in replacement. I might order the 100k version as well as @ryuuoh suggested and see if that makes a difference. Although I'm thinking a higher R in the pot might make it more sensitive.