Troodent

[withering_drums]

Ive been working on this Troodent preamp I put together a while ago. It sounds decent enough but theres some nasty noise issues. Part of it is from the phantom circuit, but even with that disconnected I am noticing noise.

So I scoped the power rails and they are dead silent. I went to trace the noise and its present all the way to input transformer.

When I remove the input transformer its dead silent. I physically removed the transformer from the case and just connected a resistor to the primary and it still adds hum, just sitting on a rubber mat on my desk. How is this possible?

Theres a couple other issues, like when I have the attenuator up past half way it just starts oscillating out of control, but Id like to sort this noise before I really dig into that.

 

[withering_drums]

Well, Im an idiot. I never grounded the audio circuit to the chassis at any point. Sorted that and it is much quieter. Solved a couple other issues at the same time...

 

[ruffrecords]

Interesting design. Where did you get the schematic from? C86 looks odd to me.

Edit: Forget that, I just found the old thread.

Cheers

Ian

 

[gyraf]

R91/92 does not compute

C86 neither, unless it has a function supporting some obscure pole in the response by bootstrapping

 

[dmp]

That project was off from the real A-Range design

 

[withering_drums]

Yeah I generally dont understand why they would pad the signal coming off the Input Xfmr or why its bootstrapped. Some sort of HF suppression or something? I just know its a fairly faithful clone of the A ranges. It seems to follow this schematic, more so than the one DMP posted anyway. Really cant remember where I got this schematic though. Likely on one the insane A range threads on here, but I started this like 5 years ago, cant remember most things. Im just excited my little DI/Pad/Phantom boards work alright!

 

[withering_drums]

Also, I believe the person who created this boards is still on GDIY. @sinestaraudio may have some insight.

 

[ruffrecords]

R91/92 does not compute

Yes, seems to be a pointless attenuator an unnecessary introducer of noise.

C86 neither, unless it has a function supporting some obscure pole in the response by bootstrapping

I found an earlier thread that show that the original Trident is a very close copy of the Revox A77 input amplifier with a transformer added at the front. I happen to have a hard copy of an original Revox A77 service manual so I have personally verified this. In the Revox circuit C86 is present but it is 3.3uF and oes appear to bootstrap the input impedance.

Cheers

Ian