ruffrecords

Re: Pre amp therapy / therapy pre
« Reply #40 on: August 10, 2017, 04:11:48 AM »
The -134dBu EIN is certainly impressive. How did you measure it?

Cheers

Ian

bump


ricothetroll

Re: Pre amp therapy / therapy pre
« Reply #41 on: August 10, 2017, 06:58:28 AM »
Hi,

I finally found what was causing the oscillation : it was the improper loading of the Lundahl mic trafo. Replaced the 220k with 9k+3nF series as recommended by Lundahl, and now the oscillations are gone.

Now, that buzz is annoying after all, I'd be happy if it was gone !

On the picture attached is a scope capture of the buzz at the output, scope settings being :
- Horizontal : 5ms/div
- Vertical : 200mV/div

The preamp gain is maxed, so that's about 83dB of gain (pentode, no feedback, gain and level maxed). Input is 150R across pin 2-3.

https://drive.google.com/file/d/0B6K-0QVHN7zUWlFVX1lBWDBXanM/view?usp=sharing

Any clue of where this comes from in the chain ? The buzz level is affected by both the gain and level control.

Thanx in advance.

Best regards.

Eric

abbey road d enfer

Re: Pre amp therapy / therapy pre
« Reply #42 on: August 10, 2017, 07:28:10 AM »
The -134dBu EIN is certainly impressive. How did you measure it?
A-weighted... so it's probably about -125/126dBu unweighted
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

ruffrecords

Re: Pre amp therapy / therapy pre
« Reply #43 on: August 10, 2017, 04:11:07 PM »
A-weighted... so it's probably about -125/126dBu unweighted

Ah, I missed the A weighting. Yes that will help a lot with 1/f noise and hum components.

Cheers

Ian

abbey road d enfer

Re: Pre amp therapy / therapy pre
« Reply #44 on: August 10, 2017, 05:46:26 PM »
Any opinion on the routing ?
There is a major mistake in your routing. You made the ground entry on the input side. As a result, the current spikes that circulate in the ground trace create a noise voltage that is directly superimposed to the input signal.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

ricothetroll

Re: Pre amp therapy / therapy pre
« Reply #45 on: August 12, 2017, 02:54:10 AM »
There is a major mistake in your routing. You made the ground entry on the input side. As a result, the current spikes that circulate in the ground trace create a noise voltage that is directly superimposed to the input signal.

I always thought grounding at the input was a good practice, as it's always done in guitar amps for example. Seems like I'm wrong ! Shouldn't be too much of a problem in such a "Class A" schematic, is it ? Anyway I joined a modified version.

BTW, is there already a thread here on this subject ? Couldn't find any. Might be a good idea to start this discussion.

For my buzz issue, I 'll try to increase B2-B3 series resistors, as I still have voltages much higher than Jonte's :
Post choke = 257V
B3 = 245V
B2 = 219V
B1 = 194V

I'll order them when back from holidays, in about 2 weeks.

To be continued

Best regards.

Eric