Mic preamp - strange high frequency roll off problem - diyAudio
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Old 3rd January 2013, 07:47 PM   #1
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Default Mic preamp - strange high frequency roll off problem

Ive been working on this preamp for over a month now with no luck. Pretty much every part of it has been rebuilt or replaced and I'm now out of ideas. The high frequencies begin to roll off at about 1k and are about 25dB down at 20k! I'm sure all of this is happening in the first gain stage as the same frequency curve is present at its cathode as at the output of the entire preamp.

Here's the basic design: EF86 DC coupled to 1/2 ECC83 DC coupled to 1/2 ECC83 AC coupled to ECC82 White cathode follower via a 1M pot.

It started off all AC coupled, but as I said, I've rebuilt the whole thing! Also I've rebuilt the EF86 stage to use a couple of diodes on the cathode and some zeners on the screen grid. No change.

If I had any hair I'd be pulling it out! Please help!
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Old 3rd January 2013, 07:54 PM   #2
cbdb is offline cbdb  Canada
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Are you sure its not whats going into the pre, like the mic, thats got the problem?
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Old 3rd January 2013, 07:56 PM   #3
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Schematic? That would make it much easier to help you.
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Old 3rd January 2013, 08:07 PM   #4
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I'm using a signal generator (a log sine sweep) in cubase and i can check whats going on in different parts of the preamp with my audio probe. I've measured an almost ruler flat response at the output of the Lundahl 1538 input transformer. The response is flat at the grid of the EF86, I even removed the grid stopper resistor to make sure!

I've just downloaded expressSCH so I'll have a go at drawing a schematic when I get home tonight.
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Old 3rd January 2013, 08:21 PM   #5
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Andyroid View Post
The high frequencies begin to roll off at about 1k and are about 25dB down at 20k! I'm sure all of this is happening in the first gain stage as the same frequency curve is present at its cathode as at the output of the entire preamp.
Sounds like the upper halve of a RIAA correction.Something around the EF86 that's a leftover from an earlier build ?
Mona
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Old 3rd January 2013, 08:42 PM   #6
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Quote:
Originally Posted by Ketje View Post
Sounds like the upper halve of a RIAA correction.Something around the EF86 that's a leftover from an earlier build ?
Mona
Not sure what that means, a quick google search tells me its an emphasis/de-emphasis EQ for vinyl. How would I have accidentally done that? It's a simple EF86 pentode gain stage with 100k anode load, 68k from B+ to screen with three 24V zeners going to the cathode (bypassed with 100n to reduce noise) and two 1N4007 diodes from cathode to ground.

From memory the B+ is 250v, screen is at 73V cathode is at 1.2V and I cant remember what voltage the anode is at. The output is taken directly from the anode and a 1M/470k voltage divider feeds the next grid.

For the time being I have removed the input transformer and there is just a 1M grid leak resistor.

This will be easier when I have the schematic drawn!
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Old 3rd January 2013, 09:42 PM   #7
Ketje is offline Ketje  Belgium
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Well perhaps not in your case,but sometimes an old preamp is used for other purposes,throwing out what is supposed not nessesairy.
Better wait for a schematic.But I have to say,the hole idea of a mic pre with a noisy EF86 (low frequency region,current repartition a/g2) and a lot of junk around it,is not the best way.
Mona
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Old 4th January 2013, 02:09 AM   #8
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Here's the preamp as it is now, minus the power supply. Would do more but it's 2 in the morning!
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File Type: gif V1d.gif (49.0 KB, 116 views)

Last edited by Andyroid; 4th January 2013 at 02:15 AM.
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Old 4th January 2013, 03:38 AM   #9
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Missed a bit of the cathode follower, don't think that part of the circuit is relevant to my problem though.

I think everything else looks ok.
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Old 4th January 2013, 10:58 AM   #10
DF96 is offline DF96  England
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If that really is the circuit you are using then poor behaviour is to be expected. Many of the resistor values are completely wrong. Miller effect will give you serious treble rolloff. That is not a mike preamp; at best it is a peculiar guitar FX box. Removing caps and replacing them with something which is far far worse is not a good way to improve a circuit.

Don't blindly imbibe the 'capacitor phobia' exhibited by some on here; instead, learn about valve circuit design.
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