UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 4th Jan 2007, 4:30 pm   #1
adibrook
Retired Dormant Member
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: Weymouth, Dorset, UK.
Posts: 377
Default Stereo amp upgrade

My Homebrew stereo amp has served me very well for a few months now. However, several problems have become apparent, and i think its about time to upgrade it to MkII. Although i am pleased with it considering how quickly i knocked it up when i blew my stereo, but improvements can be made.

Hers what its like now.

http://img.photobucket.com/albums/v4...ereoampmkI.gif

I dont know what the main smoothign caps are, theyre all in one can and the writing rubbed off.

1st i will list the problems.

1) Mains hum coming through the speakers. This seems to especially effect the tweeters.
2) VERY high gain
3)Too much trebel
4) unstable when audio input is disconnected.

The mains hum is pretty annoying, but could be worst. Its only apparent when listening to music at low volume.

The amp seems to have very high gain. I use it connected to my laptop's line output. I have to have the main volume control on my laptop as low as it can go, and also windows media players own volume control on 3%. Anything more and it goes so loud it distorts my Acoustic Solutions speakers.

Also, at this kind of gain, the s/n ratio gets pretty bad.

Althought, i gotto say, i AM impressed by the EL84 stages. They really arnt that bad. I used to think that EL84's are weak, bit not anymore. They distort my ''90W 125Wpeak'' acoustic solution speakers!

A futher probelm is lack of bass. Thats what the 2.2K resistors were put in for, but it didnt cure it totally. This is no problem because i have it all equalised out on my laptop. However, when connecting portable music devices and stuff, without good EQ fetures, this becomes very apparent.

And finally if the cable is disconnected from the headphone socket, the device squaels at maximum power! As you can imagine this can be pretty bad, especially at night. If you accidently nudge the cable enough to pull it out...it gets VERY loud .

So anyway, i'v had several ideas of my own about possible circuit improvements.

I think maybe the 470R resistor could do with being replaced with a choke.

Also, 2.2K for preamp decoupling is abit low...right? It could be about 20k and the preamps woudl still work.

What to do about gain...i dont know. I guess the EL84 is a high gain valve. I could use some 6BW6's or 6V6's or something. BUT i wasted a pack of two brand new matched marshall EL84's on this...so i may as well keep them.

I thought of goign down the famous Mullard 3 3 rout, but if the gain of the ECC83 is too high, a pentode would be even worst...right?

Another thing i thought of is lowering the grid resistors of the ECC83 significantly. Since the laptop has a low impeadence output, it would happily feed into 5K or less. And so would most modern devices. That would really lower noise pickup, and possibly the gain. But...would this effect the ECC83's in any bad way?

Also, maybe the 330K anode resistors are a tad too high?

Any other ideas are welcomed.

Thanks.
adibrook is offline  
Old 4th Jan 2007, 5:04 pm   #2
oldeurope
Retired Dormant Member
 
oldeurope's Avatar
 
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
Default Re: Stereo amp upgrade

Maybe this helps...
Darius
Attached Thumbnails
Click image for larger version

Name:	adi.png
Views:	296
Size:	42.4 KB
ID:	7551  
oldeurope is offline  
Old 4th Jan 2007, 5:59 pm   #3
adibrook
Retired Dormant Member
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: Weymouth, Dorset, UK.
Posts: 377
Default Re: Stereo amp upgrade

Thanks.

So the anodes of the EL84's are fed directly from the rectifier cathode? I also saw that in other amps, such as the mullard 3 3 i think. But...does than not make the hum worst? (obviously not if people use that technique...but could someone explain why this is better than connecting the anodes to the other end of the decoupling resistor?).

Is the cathode cap of the EL84 removed to reduce gain? Would the EL84 power stage still have thesame impressive power ouput if required (just with a bigge rinput signal i guess?)

I will try these modefications.

Thanks.
adibrook is offline  
Old 4th Jan 2007, 6:14 pm   #4
Mike Phelan
Dekatron
 
Mike Phelan's Avatar
 
Join Date: Nov 2003
Location: Near Leeds, West Yorkshire, UK
Posts: 4,609
Default Re: Stereo amp upgrade

Adi
The gain of the actual power amp should not be an issue, and it is easy to reduce it. What sort of preamp are you using, and what inputs is it feeding to?

The Mullard 3-3 is an excellent amp for its size, and as you probably have realised, is direct-coupled. Why re-invent the wheel?

If you use both stages running off the same HT rail without any decoupling/smoothing you will have problems with instability and even oscillation.

Alterations to treble (and bass) ought to be carried out in an earlier preamp stage before the first grid, or a passive network at least.

The input impedance should be fairly high or you will get frequency distortion. A very low impedance input needs either a transformer or a grounded grid stage.

Output stages do not need much smoothing because any ripple voltage on the anode and screen has little effect.

The cathode bypass cap gives less negative feedback (more gain) but only at at higher frequencies.

HTH
__________________
Mike.
Mike Phelan is offline  
Old 4th Jan 2007, 6:19 pm   #5
adibrook
Retired Dormant Member
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: Weymouth, Dorset, UK.
Posts: 377
Default Re: Stereo amp upgrade

There is no preamp. The laptop output is connected directly to that circuit.
adibrook is offline  
Old 4th Jan 2007, 6:36 pm   #6
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Stereo amp upgrade

Re: excessive treble.
It's usual to find a smallish cap. across the pri. of the O/P transformer to act as an excessive high-note cut. ISTR 0.005uF being typical - a little experimentation may be required.

By the same token, is the O/P transformer giving the correct Z-match? If not, it may explain the lack of bass.

Finally, to tackle the problem of excessive gain, noise / hum, etc., a little negative feedback from O/P to I/P should help here. Suitably chosen components in the feedback loop should enable you to determine the frequency response & gain of the amp. this way, rather than relying on the characteristics of the amp. when operating open-loop.

Just a few ideas.

Al.
Skywave is offline  
Old 4th Jan 2007, 6:57 pm   #7
oldeurope
Retired Dormant Member
 
oldeurope's Avatar
 
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
Default Re: Stereo amp upgrade

Quote:
Originally Posted by adibrook View Post
Thanks.

So the anodes of the EL84's are fed directly from the rectifier cathode? I also saw that in other amps, such as the mullard 3 3 i think. But...does than not make the hum worst?
Thanks.
The EL84 is a pentode. Like a transistor collector the plate current of a pentode is independent from the plate voltage if the plate voltage is higher than say 50 Volts. So riples do mot make a current change, no hum.
Darius
oldeurope is offline  
Old 4th Jan 2007, 9:08 pm   #8
adibrook
Retired Dormant Member
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: Weymouth, Dorset, UK.
Posts: 377
Default Re: Stereo amp upgrade

Ok

So i will preform these modefications (modefied parts marked in red)

http://img.photobucket.com/albums/v4...reoampmkII.gif

I need to change the rectifier valve socket. Its an old wafer socket and its falling appart.

I thought about just copying the 3 3 amp, but i dont have any of the really high resistors around the EF86. I could buy some, but i woudl like to have this amp workign again today...
adibrook is offline  
Old 4th Jan 2007, 11:01 pm   #9
adibrook
Retired Dormant Member
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: Weymouth, Dorset, UK.
Posts: 377
Default Re: Stereo amp upgrade

Well...i did all the mods. Thanks for the advice.

The amp does sound somewhat nicer, and the volume doesnt have to be so low, so alot less noise.

Also, i changed the rectifier socket. Now the rectifier doesnt fall out all the time. So hopefully now i can just leave the amp to preform its task with not alot more interfearence from me.
adibrook is offline  
Old 5th Jan 2007, 9:19 am   #10
peter_sol
Octode
 
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
Default Re: Stereo amp upgrade

Hi. You should have a 1 meg volume control at the input grid instead of the 1 meg resistor otherwise you will have what appears to be too much gain and hum on disconnecting the input
What you have done is 47K resistors which have reduced the input impedance thereby reducing the hum pickup.
It is no good relying on the volume adjustment on the signal source.
Regards
Peter.
peter_sol is offline  
Old 5th Jan 2007, 9:29 am   #11
oldeurope
Retired Dormant Member
 
oldeurope's Avatar
 
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
Default Re: Stereo amp upgrade

Quote:
Originally Posted by adibrook View Post
Well...i did all the mods. Thanks for the advice.

The amp does sound somewhat nicer, and the volume doesnt have to be so low, so alot less noise.

Also, i changed the rectifier socket. Now the rectifier doesnt fall out all the time. So hopefully now i can just leave the amp to preform its task with not alot more interfearence from me.
Good morning,
ok, don't forget to change the 330K into 220K. If you still have too much gain, let me know.

Darius
oldeurope is offline  
Old 5th Jan 2007, 9:35 am   #12
adibrook
Retired Dormant Member
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: Weymouth, Dorset, UK.
Posts: 377
Default Re: Stereo amp upgrade

Oh yeah. I changed the anode resistors, but forgot to change them on the diagram.

When i can get some double pots i will sort out the volume control.

For now its working well tho. Thanks
adibrook is offline  
Old 5th Jan 2007, 10:57 am   #13
Merlin
Retired Dormant Member
 
Merlin's Avatar
 
Join Date: Apr 2006
Location: Shrewsbury, Shropshire, UK.
Posts: 67
Default Re: Stereo amp upgrade

Personally I don't like the changes that have been suggested. I would DEFINATELY have kept that 470R resistor instead of running the EL84's right off the restifier- you'll get MASSIVE ripple voltage there.

What I would have done is to reduce the anode resistor on the ECC83 to 100k to reduce gain. Remove the cathode cap from the ECC83 then apply global feedback from the speaker to cathode to reduce gain and increase linearity (this is a HiFi stereo after all) and use the feedback to also compensate for the treble.

Plus you have no resistance in series with the rectifier- you're absolutely raping it. I would expect the thing to burn out very soon, especially now you're running the EL84's right off it.
Merlin is offline  
Old 5th Jan 2007, 11:22 am   #14
oldeurope
Retired Dormant Member
 
oldeurope's Avatar
 
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
Default Re: Stereo amp upgrade

Hi, having no negative feedback has some interesting advantages.
What kind of speaker do you use?

If you don't block the kathode of a triode, the kathode resistance is multipled by micro to the plate output. This is really bad. A not blocked kathode makes only sense with pentodes (and transistors). Good designed triode amps have blocked kathodes! Test the ECC81, It may sound better in your application. Please note the 2K2 resistors at the triode grids. picture in #15

Darius

Last edited by oldeurope; 5th Jan 2007 at 11:27 am. Reason: pic in #15
oldeurope is offline  
Old 5th Jan 2007, 11:24 am   #15
oldeurope
Retired Dormant Member
 
oldeurope's Avatar
 
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
Default Re: Stereo amp upgrade

Forgott the pic. ...
Attached Thumbnails
Click image for larger version

Name:	adi.png
Views:	196
Size:	21.7 KB
ID:	7567  
oldeurope is offline  
Old 5th Jan 2007, 11:31 am   #16
Merlin
Retired Dormant Member
 
Merlin's Avatar
 
Join Date: Apr 2006
Location: Shrewsbury, Shropshire, UK.
Posts: 67
Default Re: Stereo amp upgrade

Quote:
Originally Posted by oldeurope View Post
The EL84 is a pentode. Like a transistor collector the plate current of a pentode is independent from the plate voltage if the plate voltage is higher than say 50 Volts. So riples do mot make a current change, no hum.
Not true, unless you were operating it into an infinite load resistance. In fact a pentode is VERY susceptible to power supply ripple (when single ended). Because of its very high anode impedance it has almost zero PSRR, so all that mains ripple is fed directly into the output transformer and speaker!
Merlin is offline  
Old 5th Jan 2007, 11:45 am   #17
oldeurope
Retired Dormant Member
 
oldeurope's Avatar
 
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
Default Re: Stereo amp upgrade

Quote:
Originally Posted by Merlin View Post
Not true, unless you were operating it into an infinite load resistance. In fact a pentode is VERY susceptible to power supply ripple (when single ended). Because of its very high anode impedance it has almost zero PSRR, so all that mains ripple is fed directly into the output transformer and speaker!
Hi Merlin, I don't understand infinite load resistance and PSRR.
Please explain.
Thanks, Darius
oldeurope is offline  
Old 5th Jan 2007, 11:59 am   #18
Merlin
Retired Dormant Member
 
Merlin's Avatar
 
Join Date: Apr 2006
Location: Shrewsbury, Shropshire, UK.
Posts: 67
Default Re: Stereo amp upgrade

PSRR = Power Supply Ripple Rejection.

Ok, imagine you have a simple single ended amplifier stage, something like this: http://oldradio.qrz.ru/tubes/foreign/01/EL34.gif
and imagine the power supply isn't steady, it has some 100Hz ripple voltage.

You know how a potential divider works right? Well if you imagine the valve is really a resitor:- you now have a potential divider created by the load (in this case the output transformer) and the resistor (the valve). You've basically got a volume control with a ripple signal coming in from the power supply.
If the impedance of the valve is low (like a triode), the ripple voltage is attenuted and most of it won't make it into the audio signal (good PSRR), as if the volume control is turned down.

If the impedance of the valve is high (like a pentode), then almost ALL the ripple voltage appears directly across the valve so all of it gets mixed with the audio signal! Therefore pentodes need better filtered power supplies than triodes for a given noise performance.
Merlin is offline  
Old 5th Jan 2007, 12:05 pm   #19
ppppenguin
Retired Dormant Member
 
ppppenguin's Avatar
 
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
Default Re: Stereo amp upgrade

Quote:
Originally Posted by Merlin View Post
If the impedance of the valve is high (like a pentode), then almost ALL the ripple voltage appears directly across the valve so all of it gets mixed with the audio signal! Therefore pentodes need better filtered power supplies than triodes for a given noise performance.
Maybe I'm mistaken, but the ripple voltage across the valve is not important. The voltage that matters is the ripple voltage across the output transformer. If it's almost all across the valve then almost none is across the transformer.
ppppenguin is offline  
Old 5th Jan 2007, 12:16 pm   #20
Merlin
Retired Dormant Member
 
Merlin's Avatar
 
Join Date: Apr 2006
Location: Shrewsbury, Shropshire, UK.
Posts: 67
Default Re: Stereo amp upgrade

Quote:
Originally Posted by ppppenguin View Post
Maybe I'm mistaken, but the ripple voltage across the valve is not important. The voltage that matters is the ripple voltage across the output transformer. If it's almost all across the valve then almost none is across the transformer.
It is a bit counter intuitive yes. The actual ripple signal appears across both or course, but the ratio of the two impedances determines the amplitude of the ripple signal. When ra is high, so too is the ripple amplitude, which is fed into the OT.
Merlin is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 4:30 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.