# Two Sticks of Derm high frequency noise?



## Redredbettyz (May 1, 2020)

Just built this up and it sounds pretty sweet. It's a nice mid-high gain one knobber with tons of output volume. I have noticed that there is a strange high frequency noise/crunch when I start cranking the volume pot much further than unity volume. Is there a certain hfe I should be looking for with the transistors? I did have to use one 10uf tantalum cap while I'm waiting for my next shipment of parts, but didn't know if that might be an issue. Anyone else build one up and have this issue?


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## Redredbettyz (May 8, 2020)

I think I got mine figured out. If anyone experiences the same problem, try out a lower gain transistor than the 2n3904 as the center transistor on the PCB.


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## radioteacher (May 13, 2020)

Redredbettyz said:


> I think I got mine figured out. If anyone experiences the same problem, try out a lower gain transistor than the 2n3904 as the center transistor on the PCB.


 What transistor would you recommend?


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## Liricus (May 19, 2020)

Same issue here, high pitch "squealing" noise, although not very loud, when knob set beyond 12 o'clock. Let us know which transistor substitution worked the best for you.


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## Nostradoomus (May 20, 2020)

Just try and sort your 2n3904 until you get one around 200hfe, it happens once in awhile. I put an S9018 (190hfe) in there and Q3, mpsa18 in Q1. Sounds pretty good and no weird frequencies.


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## jspake (May 22, 2020)

glad to know i'm not the only one who is having this issue, although for me the squeal-ish thing only happens at the last little bit before maxing out. i'll try the changing out the transistors as recommended. god bless the search function here.


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## jspake (May 23, 2020)

actually, i do need some help here. i went through maybe 10-15 2n3904 units and still got the shriek/squeal which is very annoying. i don't have a way to test hfe that i know of right now. i ordered a component tester that should be here next week some time. is there another component that can be switched out to reduce this noise? or another npn type?


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## Redredbettyz (May 25, 2020)

radioteacher said:


> What transistor would you recommend?


I used a BC109 with Hfe of 141 for the middle transistor that fixed my issue.


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## Nostradoomus (May 25, 2020)

2n2222 should fit the bill as well.


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## radioteacher (May 25, 2020)

I have been building kits for over four decades but still have more to learn.  Let's all learn from these mistakes.

Testing the pbc outside a case and shielding can cause undesired effects.
Even if the documentation is not perfect, look it over very closely.  
Notice things like the unlabeled holes used to ground the input/output jacks.
DO NOT hook up the center power pin to 9 Volts Positive.  Even though every prior project you built did.  
Hey designers, thank you for the protection diode.


Lower gain transistors might be better for your style.
This circuit has enough gain to crush the clean channel of my amp with a low-output telecaster lead pickup.

RT


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