Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

How to measure the output impedance of a valve preamp?
How to measure the output impedance of a valve preamp?
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th May 2019, 03:36 PM   #41
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
Quote:
Originally Posted by JMFahey View Post
Not sure the OP means to straight load a triode stage output with a low value resistor to lower its output impedance, the proper way is to use a voltage divider so tube plate only "sees" the upper branch of the divider.

IF he uses, say, a 100k:1k divider plate will "see" 100k but divider output impedance will be roughly 1k .
Of course, thereīs a large attenuation involved.

At least, thatīs what he did in his schematic, where tube drives a tone stack and volume control and then a 3k3 resistor.
This is a bad idea because you throw away a lot of gain, so you need lots of gain to throw away which means lots of distortion.

The way to do it is design for lowish Zout, or add a cathode follower. And a power amp with 10k Zin is not a low input impedance, so no need for heroic measures at the sending end. If you feed a 10k Zin from a 5k Zout you lose only 3dB. That's OK.

Jan
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.
  Reply With Quote
Old 27th May 2019, 04:09 PM   #42
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
Originally Posted by jan.didden View Post
This is a bad idea because you throw away a lot of gain, so you need lots of gain to throw away which means lots of distortion.
Well, he IS looking for distortion, itīs a Guitar pedal.
And yes, he uses tons of gain precisely to overdrive a tube stage.
After which heīll have some 90V RMS "clean" signal or some 250Vpp ... in any case he *must* attenuate that signal to drive anything.
Quote:
The way to do it is design for lowish Zout, or add a cathode follower.
After which heīll still have the 250Vpp signal which must be attenuated anyway.


Quote:
Quote:
Ideally I would need an attenuation 100:1while reducing the output impedance at the same time. So, a full voltage divider is a proper way to do that, right? Where would you suggest to place it - before or after the master volume? Thanks.
We are on post #40 by now, lots of water passed under the bridge, please re-confirm what preamp will you use and what load will you drive.
We need input impedance and signal level required.
My point being that probably the Master Volume will be part of the attenuator and not 1M any more.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.

Last edited by JMFahey; 27th May 2019 at 04:15 PM.
  Reply With Quote
Old 27th May 2019, 06:29 PM   #43
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
Quote:
Originally Posted by JMFahey View Post
Well, he IS looking for distortion, itīs a Guitar pedal.
And yes, he uses tons of gain precisely to overdrive a tube stage.
After which heīll have some 90V RMS "clean" signal or some 250Vpp ... in any case he *must* attenuate that signal to drive anything.
OK, yes, that'll change things ...
Funny guys, these guitar people ;-)

Jan
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.
  Reply With Quote
Old 28th May 2019, 04:26 AM   #44
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by jan.didden View Post
Funny guys, these guitar people ;-)
Jan
I was interested in electronics (mostly radio) as a child, then in electronics (mostly Hi-Fi) as a teenager and into my twenties. In my twenties I also taught myself to play acoustic guitar, then electric guitar, and finally, married and with a mortgage, I got serious about guitar playing in my forties.

So now I am one of those "guitar people", but with almost a life-long background in "proper" analogue electronics, particularly audio.

It was a heck of an unlearning curve, I can tell you! Almost everything I knew about audio electronics no longer applied where electric guitar was concerned. Low distortion sounds bad. Opamps sound bad. Speakers with a flat frequency response sound bad. Tweeters sound bad. Low output impedance (to the speaker) sounds bad.

Because I had the technical background, I could, with time, tell the B.S. from the reality. All the things I listed in the previous paragraph really are engineering and scientific reality when electric guitar amplification is involved - there really are good technical reasons for them!

(For example, opamps sound bad because with guitar, they will be driven into clipping. Because opamps are designed with near-infinite voltage gain, when used for audio amplification, they are always set up with large amounts of negative feedback. And this means that when clipping does occur, it is harsh and very abrupt. So as you turn up the level, an e-guitar through an op-amp goes from too-clean and boring to too-harsh and nasty, with nothing in between!

As has been said before, we really shouldn't think of a guitar amplifier as an audio amplifier. We should think of it as a part of the instrument - it is a signal processor, designed to take an instrument that generates rather boring sounds on its own, and modify the signal so much that what comes out is almost unrelated to what went it.


-Gnobuddy
  Reply With Quote
Old 28th May 2019, 06:34 AM   #45
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
To give some practical example: this is a killer early Marshall Guitar amplifier ,18W, **obviously** overdriven, no need for an analyzer to detect that:
YouTube

Please listen to it first, only then click the link below to see its waveform when overdriven.

Please do not cheat so as not to be prejudiced

https://robrobinette.com/images/Guit...driveScope.jpg

also same but somewhat *more* overdriven at bottom:

https://www.paulamps.com/OutputsNoBuzzCap.jpg

another example, here bottom waveform is measured across speaker terminals, top one is measured at a tube *before* the power ones, which is also clipping on its own, go figure:

https://www.paulamps.com/CrossOverSolution.jpg
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
  Reply With Quote
Old 28th May 2019, 06:39 AM   #46
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
Quote:
Originally Posted by Gnobuddy View Post
I was interested in electronics (mostly radio) as a child, then in electronics (mostly Hi-Fi) as a teenager and into my twenties. In my twenties I also taught myself to play acoustic guitar, then electric guitar, and finally, married and with a mortgage, I got serious about guitar playing in my forties.

So now I am one of those "guitar people", but with almost a life-long background in "proper" analogue electronics, particularly audio.

It was a heck of an unlearning curve, I can tell you! Almost everything I knew about audio electronics no longer applied where electric guitar was concerned. Low distortion sounds bad. Opamps sound bad. Speakers with a flat frequency response sound bad. Tweeters sound bad. Low output impedance (to the speaker) sounds bad.

Because I had the technical background, I could, with time, tell the B.S. from the reality. All the things I listed in the previous paragraph really are engineering and scientific reality when electric guitar amplification is involved - there really are good technical reasons for them!

(For example, opamps sound bad because with guitar, they will be driven into clipping. Because opamps are designed with near-infinite voltage gain, when used for audio amplification, they are always set up with large amounts of negative feedback. And this means that when clipping does occur, it is harsh and very abrupt. So as you turn up the level, an e-guitar through an op-amp goes from too-clean and boring to too-harsh and nasty, with nothing in between!

As has been said before, we really shouldn't think of a guitar amplifier as an audio amplifier. We should think of it as a part of the instrument - it is a signal processor, designed to take an instrument that generates rather boring sounds on its own, and modify the signal so much that what comes out is almost unrelated to what went it.


-Gnobuddy
Thanks for the explain. Fascinating!

Jan
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.
  Reply With Quote
Old 28th May 2019, 11:39 AM   #47
i3alan is offline i3alan  Canada
diyAudio Member
 
Join Date: May 2019
Quote:
Originally Posted by JMFahey View Post
We are on post #40 by now, lots of water passed under the bridge, please re-confirm what preamp will you use and what load will you drive.
We need input impedance and signal level required.
My point being that probably the Master Volume will be part of the attenuator and not 1M any more.
I would like to make the output compatible with as many devices as possible. So the preamp could be connected to the front of guitar amp, to the fx return input of an amp, line level audio input or daisy chained with another guitar pedals. So I would guess the input impedance of the load would be >10K, the signal level in the range of -10dBv to +4dBu. Please see the schematics of my last two gain stages and the output. As I mentioned, I would like to attenuate the signal, say 100:1, and get a lower output impedance.
So, with the master volume pot being used as a part of the voltage divider, where would you place the rest of the voltage divider circuit - before or after the pot?
Attached Images
File Type: png tube preamp.png (21.0 KB, 30 views)

Last edited by i3alan; 28th May 2019 at 12:03 PM.
  Reply With Quote
Old 29th May 2019, 02:08 AM   #48
PRR is online now PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
Sketch:
Attached Images
File Type: gif i3alan-1.gif (10.9 KB, 21 views)
  Reply With Quote
Old 29th May 2019, 02:30 AM   #49
i3alan is offline i3alan  Canada
diyAudio Member
 
Join Date: May 2019
Quote:
Originally Posted by PRR View Post
Sketch:
Thank you so much!
  Reply With Quote
Old 29th May 2019, 06:02 AM   #50
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
Quote:
Originally Posted by PRR View Post
Sketch:
I think the switch is not corrected correctly. The label on the switch connection CH2_Vol should be CH1_Vol.
Also, the Zout is dependent on the pot setting.
ALL schemes that have the pot at the end will have Zout that varies with pot settings.

Jan
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.

Last edited by jan.didden; 29th May 2019 at 06:10 AM.
  Reply With Quote

Reply


How to measure the output impedance of a valve preamp?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Please help how to measure/guage output of guitar preamp garybdmd Instruments and Amps 21 25th October 2016 01:51 AM
12AU7 Valve output impedance. nigelwright7557 Tubes / Valves 18 7th August 2016 10:50 PM
How to measure amplifier`s output impedance wrt Frequency? rhythmsandy Solid State 10 10th September 2014 07:06 PM
How to measure the output impedance of the DAC chip ergo Solid State 2 9th June 2004 07:07 AM
How do I calculate (or measure) my amp's output impedance? Saurav Tubes / Valves 22 27th September 2003 03:55 AM


New To Site? Need Help?

All times are GMT. The time now is 03:35 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki