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Help with what to check next - pre-amp distorting at high/full volume

MITEE

May 16, 2022
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Help with what to check next - pre-amp distorting at high/full volumeI have a preamplifier which has a problem, I am looking for some suggestions on what to check next to diagnose.

Problem set:

Output distorts on ALL channels at high volume and when set to "direct" and volume is bypassed

Output is normal at lower volumes, no changing of dials or inputs changes the distortion

Distortion is heard on peaks in music passages

I have pulled the 4 output transistors and checked them, they are fine. I was pretty sure it was going to be one of these but it wasn't.

What would be the most likely culprit if not the transistors? I didn't think caps distorted the way I described here, am I mistaken?
 

Bluejets

Oct 5, 2014
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You will need to supply a lot more detail on your equipment.
Not unusual for amps to distort at max volume.
 

MITEE

May 16, 2022
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You will need to supply a lot more detail on your equipment.
Not unusual for amps to distort at max volume.

It shouldn't distort when it has a "direct" button to bypass volume control. By design that's meant to provide a max signal.

It's a Theta, so there is nothing out there in terms of schematics or service manuals, that's why I asked the question generically for ideas for areas to look at. It's also not the kind of gear a lot of people will say they have opened up. I was pretty sure I was going to find the fault in the transistors, I desoldered them, but using a generic testers they all came out clean.
 

crutschow

May 7, 2021
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Likely it's a design problem with the amps not having enough dynamic range, so they clip the signal at high levels.
Don't see how you can solve that without having a circuit schematic.
Does it have an internal power supply?
 

Bluejets

Oct 5, 2014
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It's a Theta, so there is nothing out there in terms of schematics or service manuals,

Perhaps but are you using say, another preamp or coupled together some gear of your own..??
Independant power supplies, what are you feeing into it? Line or signal levels?

So far all we know is, it's a box with an output that is distorting.
 

Harald Kapp

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Is this a new issue with that amp or has it existed all along?
If this is an older unit, check the electrolytic capacitors in the power supply. These tend to lose capacity with time so they lose the ability to supply peak current.
 

MITEE

May 16, 2022
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Likely it's a design problem with the amps not having enough dynamic range, so they clip the signal at high levels.
Don't see how you can solve that without having a circuit schematic.
Does it have an internal power supply?

It does. All the voltages are numbers that we're used to seeing, I cant recall specifics. There are 4 separate transformers in the unit, one for each tap. They did not spare expense.

Is this related to signal strengths that are too high on the input? I have this problem with some old gear, but this unit isn't that old. I am using Sonos at the input. In that case attenuation would solve the problem permanently by adding an attennuator, perhaps drill in a switch on the back or something.

Perhaps but are you using say, another preamp or coupled together some gear of your own..??
Independant power supplies, what are you feeing into it? Line or signal levels?

So far all we know is, it's a box with an output that is distorting.

Sonos. I can try a signal generator, but as I said, if I lower the input signal the distortion stops.

The question is if this is a design issue or a fault on the board.
 

MITEE

May 16, 2022
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Is this a new issue with that amp or has it existed all along?
If this is an older unit, check the electrolytic capacitors in the power supply. These tend to lose capacity with time so they lose the ability to supply peak current.

I have owned it about 2 years but never used it, and happened to not use it on the direct setting all this time. The answer is I have no idea.

I have tested the transistors, they are ok. I suppose I should change all the caps too? I hate to be that "ALL CAPS" guy like you see suggest changing capacitors at the drop of a hat. The few I did test were fine and I started looking other places.

In the event that it is a design flaw, some nice high tolerance vishays or something might minimize the issue.

Per the change above I suggested, what would be the ideal method of attenuation here? Should I go right for the input signal? I hate diminishing it so early on, but without a schematic, its hard to account for everything. They are beautifully built inside, if you ever get a chance, open one.
 

MITEE

May 16, 2022
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No need to swap good capacitors.

Its a lot of work to pull and test them all. I can just toss the good ones in the good but old pile I go to when I am out of new caps.

That said, old board. Traces start lifting.

Should I just attenuate it and call it a "feature" or do you think its worth trying to find something w/o a schematic?
 

Martaine2005

May 12, 2015
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I am using Sonos at the input.
Can you clarify what you mean here?.
Sonos has an app. Sonos has speakers.
Do you mean a mobile phones headphone output to pre-amp input and then output to an amp?


Martin
 

MITEE

May 16, 2022
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Can you clarify what you mean here?.
Sonos has an app. Sonos has speakers.
Do you mean a mobile phones headphone output to pre-amp input and then output to an amp?


Martin

No, the coax output of the Sonos connect

I dont use any of that new sonos stuff
 

Harald Kapp

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No, the coax output of the Sonos connect
???
Acc. to the manual the Sonos Connect has 3 types of output:
- Toslink (optical)
- Coax (digital)
- Cinch (analog)
I'm sure you must be using the Cinch output to connect to your analog preamp.

The Sonos Connect has two options for volume control:
- fixed
- variable
The fixed option is meant to be used when the volume control happens in the connected amplifier. This is, I think, your scenario.
The variable option is meant to be used with an amplifier without volume control. In this case the Sonos controls the volume.
You can try to use the variable option with your setup, too. Then you can reduce the output level of the Sonos below the value where your preamp starts to distort. Then leave teh Sonos as this new "fixed" setting and control the volume via your preamp.
 

MITEE

May 16, 2022
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???
Acc. to the manual the Sonos Connect has 3 types of output:
- Toslink (optical)
- Coax (digital)
- Cinch (analog)
I'm sure you must be using the Cinch output to connect to your analog preamp.

The Sonos Connect has two options for volume control:
- fixed
- variable
The fixed option is meant to be used when the volume control happens in the connected amplifier. This is, I think, your scenario.
The variable option is meant to be used with an amplifier without volume control. In this case the Sonos controls the volume.
You can try to use the variable option with your setup, too. Then you can reduce the output level of the Sonos below the value where your preamp starts to distort. Then leave teh Sonos as this new "fixed" setting and control the volume via your preamp.


Its a DSPre which is a dac/preamp combo. Its has a dac chip onboard, but the signal out from there looks clean. The sonos's coax is the input to the Theta.

You are correct, though, I can fix the volume of the sonos, I thought it would be good to future proof the unit with a way to attenuate the input from the Sonos. If Theta were to stand alone as a DAC, it couldn't any longer, as signal strengths are not going to go down ever.
 
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