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

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Has anyone used an opamp to split phase for a tube amp?
Has anyone used an opamp to split phase for a tube amp?
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 6th April 2016, 09:19 PM   #31
SpreadSpectrum is offline SpreadSpectrum  United States
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Also, something that works well with little tubes like an EL84, is to use LM317s instead of cathode resistors.

I had an old Fisher amp that used 7591s (similar drive and bias requirements to the EL84) that shared a common cathode resistor. I had red-plating issues since it was hard to get four really well-matched tubes.

I removed the common resistor and put four individual LM317s, wired in current-source mode, one for each tube. I bypassed the LM317s with caps so that they are voltage sources at AC (just like you would do with a cathode bias resistor). Now all four tubes automatically bias themselves each time I power up and there are no more DC imbalance issues.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 7th April 2016, 04:03 AM   #32
repair guy is offline repair guy
diyAudio Member
 
Join Date: Jan 2013
You guys were right about eliminating the 12AU7 stage, it was adding alot of noise, and just sounded odd. I rebuilt it in stereo with a TL084 quad opamp and fixed the noise and ground issues. Now I'm driving the outputs directly from the opamp and it sounds even better. Now it has some real detail and depth to the sound on headphones. I got caught up listening to CD's on it for an hour with no NFB. Still have some difficulties with adding NFB. It doesn't seem to actually change any of the ringing on the scope trace, just lowers the gain, makes the square wave sharper and extends the frequency response flat to 80,000hz, then it fades away quickly at 100,000hz.

Right now there are no capacitors in the circuit, making it a direct coupled amp. I would add them later, don't want a opamp problem to fry my tubes.

Here are some scope pics without any NFB. Didn't change the gain, just square wave input. The ringing is not from the IC, it must be from the tubes or output transformer.

1k at 1W, no NFB
Click the image to open in full size.

5k at 1W, no NFB
Click the image to open in full size.

10k at 1W, no NFB
Click the image to open in full size.

20k at 1W, no NFB
Click the image to open in full size.

50k at 1W, no NFB
Click the image to open in full size.
  Reply With Quote
Old 7th April 2016, 04:27 AM   #33
BinaryMike is offline BinaryMike  United States
diyAudio Member
 
BinaryMike's Avatar
 
Join Date: Apr 2010
Location: Oregon
The ringing is probably an output transformer resonance. It's so mild that you could be forgiven for leaving it be. The usual fix is a snubbing network attached to OPT primary, typically one series RC network across the whole winding, but sometimes more complex.

There's no apparent need for feedback to flatten response at the high end. It's considered mandatory in pentode amps to reduce output impedance, however, and we don't have evidence about low-frequency flatness or distortion yet.
  Reply With Quote
Old 7th April 2016, 04:59 AM   #34
repair guy is offline repair guy
diyAudio Member
 
Join Date: Jan 2013
Quote:
Originally Posted by BinaryMike View Post
The ringing is probably an output transformer resonance. It's so mild that you could be forgiven for leaving it be. The usual fix is a snubbing network attached to OPT primary, typically one series RC network across the whole winding, but sometimes more complex.

There's no apparent need for feedback to flatten response at the high end. It's considered mandatory in pentode amps to reduce output impedance, however, and we don't have evidence about low-frequency flatness or distortion yet.

The resonance is at about 25,000hz. I know about the output impedance, but on headphones it sounds great, but on speakers it would suck. Probably when this thing is done I'll have it switch off the NFB for headphones. It still is odd to look down at the single IC doing it all, preamp and phase inversion. There's so few components there.

What values should I start with for the series RC network?

It's a 5k to 8ohm transformer.
  Reply With Quote
Old 7th April 2016, 10:21 PM   #35
BinaryMike is offline BinaryMike  United States
diyAudio Member
 
BinaryMike's Avatar
 
Join Date: Apr 2010
Location: Oregon
I usually hook up resistor and capacitor substitution boxes across the primary and just tweak for the best squarewave result. Writing in Glass Audio issue 6 of 1995, Jerry Boncer suggested making R equal to end-to-end primary Z and making capacitive reactance equal to R at the ring frequency or a bit lower. That's probably as good a starting point as any.
  Reply With Quote
Old 8th April 2016, 05:23 PM   #36
JZatopa is offline JZatopa  United States
diyAudio Member
 
Join Date: May 2003
Location: Chicago
Correct me if I am wrong but those traces look like they have better phase response than I have seen in most PP amplifiers. Do you have a way to do a phase, IMD and THD graph for this circuit?


Its funny how things have changed in the past few years in tube audio. I proposed something like this a few years ago and got a mixed response. Now it looks like everyone is starting to move to the, "what ever works best is best" camp; and I am glad to see it.
__________________
Knowledge is Power & Applied Knowledge is Freedom,
There is no higher religion than truth
  Reply With Quote
Old 8th April 2016, 06:51 PM   #37
Eli Duttman is online now Eli Duttman  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
Now it looks like everyone is starting to move to the, "what ever works best is best" camp; and I am glad to see it.
AMEN! Pragmatism should be in the forefront. I'd use a lump of galena, if it works well. Galena may be OK for an AM envelope detector, but has (IMO) no use in HIFI.
__________________
Eli D.
  Reply With Quote
Old 9th April 2016, 04:11 PM   #38
stocktrader200 is offline stocktrader200  Canada
diyAudio Member
 
Join Date: Jan 2008
Location: Calgary
the silicon will work great as the VAS stage but almost all the tubey sound will be lost.
A tube preamp (VAS) should be use to drive the SS stage for best results.
ie. 12AX7 driving a SS 2 - 5 ma mosfet concertina driving the output stage.

Last edited by stocktrader200; 9th April 2016 at 04:16 PM.
  Reply With Quote
Old 9th April 2016, 04:38 PM   #39
Tubelab_com is offline Tubelab_com  United States
diyAudio Member
 
Tubelab_com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
Galena may be OK for an AM envelope detector, but has (IMO) no use in HIFI.
Wait a minute.....can't we distribute "magic" Galena crystals to absorb the evil sound created by the silicon?

Quote:
Its funny how things have changed in the past few years in tube audio.
The early response to PowerDrive (CCS loaded triode driver, mosfet output tube grid drive) was decidedly negative about 10 years ago. I even got some email suggesting my name be changed to Transistorlab. About 400 TSE boards have been sold since then and people seem to like them (PowerDrive and DHT's).

I have some direct coupled hybrid tube / SS amps in the works, including a microprocessor controlled version.....not sure the tube world is quite ready for the Arduino / tube hybrid yet. How about the guitar amp with tubes and an Intel Core I5......That one is being built right now.
__________________
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
  Reply With Quote
Old 9th April 2016, 05:04 PM   #40
Mr_Zenith is offline Mr_Zenith  United States
diyAudio Member
 
Mr_Zenith's Avatar
 
Join Date: Aug 2009
Location: KC Metro
Has anyone used an opamp to split phase for a tube amp?
Quote:
Originally Posted by tubelab.com View Post
Wait a minute.....can't we distribute "magic" Galena crystals to absorb the evil sound created by the silicon?
Do I hear another "AY-men"?

I myself think that tubes are the coolest thing since individually-wrapped cheese slices, but I never did understand the whole purist mentality. The same people who bleat on incessantly about the "evils o' silicon" in a tube amp seem to have no such hangup when it comes to silicon in the signal path of their chosen source - be it a CD player, an iPad, or even a turntable.

I tend to view a well-designed hybrid as a creation that combines the best attributes of both technologies. Like Eli said, whatever works. As for the Arduino/tube hybrid: I say bring it on!
  Reply With Quote

Reply


Has anyone used an opamp to split phase for a tube amp?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
Split supply, arrangement of gain and phase splitter tubes DrewP Tubes / Valves 4 26th July 2009 05:18 PM
Can I phase split like this? Sherman Tubes / Valves 14 16th June 2007 11:04 PM
Looking for voyd split phase circuit dhole Analogue Source 0 25th March 2005 08:17 PM
Looking for good book about phase split alan_982002 Tubes / Valves 2 29th March 2004 12:43 AM


New To Site? Need Help?

All times are GMT. The time now is 11:48 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki