[00940]

Max power was estimated by Dsavitsk at about 750mW, I believe.  I would be surprised if some OTL amps exceed that with low-impedance loads, but I suppose if the tubes were very powerful in their own right, it might.

 
It's not very difficult to calculate. If B+ is about 275Vdc, Va about 200Vdc and the CCS needs about 10V to operate, max positive swing is 65V. Since the edcor has a 10K-32 ratio, it translates into almost a 18:1 voltage ratio. Hence, into a 32r load, max positive swing is 65/18= 3.6. That's about 2.5Vrms. From there, rms power is about 200mW into 32r.
 
This doesn't take into account DCR losses, so real max power is more like 180mW.  Which is way more than most OTL into 32R. To put things in perspective, the optimized Morgan Jones is only 10mW into 32r at clipping.
 
 
edit: silly me... power could be directly calculated from the 65V into 10K... but the result is the same.

 
edit 2: that's absolute max power. You'd have to actually measure the amp to have the power at 1%thd, which is a common figure to quote the power at. Sims say 20mW (0.8Vrms into 32r) but it's at best a ballpark figure, at worst completely off.

 

[tomb]

 

Max power was estimated by Dsavitsk at about 750mW, I believe.  I would be surprised if some OTL amps exceed that with low-impedance loads, but I suppose if the tubes were very powerful in their own right, it might.

 
It's not very difficult to calculate. If B+ is about 275Vdc, Va about 200Vdc and the CCS needs about 10V to operate, max positive swing is 65V. Since the edcor has a 10K-32 ratio, it translates into almost a 18:1 voltage ratio. Hence, into a 32r load, max positive swing is 65/18= 3.6. That's about 2.5Vrms. From there, rms power is about 200mW into 32r.
 
This doesn't take into account DCR losses, so real max power is more like 180mW.  Which is way more than most OTL into 32R. To put things in perspective, the optimized Morgan Jones is only 10mW into 32r at clipping.
 
 
edit: silly me... power could be directly calculated from the 65V into 10K... but the result is the same.

Thanks for the calculation!  I think my memory betrayed me, big time.  It must've been 250mW that Dsavitsk told me.  He was estimating B+ at the time, though, at about 250-300V, but didn't know exactly where.

 

 

[00940]

Yep, with a B+ at 300V, 250mw would make perfect sense. I took the 275V figure from that post.
 
I might have been rounding some numbers down rather than up too.
 
edit (again): if someone wanted more power, it could be possible to use the 7K winding rather than the 10K one. You get almost 50% more power that way. It would be ok with the E90CC but not the 6J6, as the E90CC have a much lower anode resistance.
edit (00940 is a serial editor): going from 200mW to 300mW is in practice negligible. I don't actually advice hacking the board for that; it was just a theoretical remark.

 

[tomb]

serial editors unite!

 

[tdubl07]

Ok guys, just got to work and plugged in my dt-990 pro's. There is no way the sound I'm getting is how it's supposed to sound. I can very easily max out the volume pot and get to barely reasonable levels. Lots of distortion of too. No bass at all. Where shall I begin troubleshooting tonight? Am I correct in saying the output coupling caps are non polarized right? Just the stock Solen's, but there were no markings on them. I'm getting 272V at B+. And the choke is like a resistor correct? So no right or wrong way to put that in (ie backwards). All of my solder joints look real good. It seems like I have something in the wrong spot, or omitted something. All 4 Led's light up from the get go as soon as I turn it on, heaters on the tubes get nice and orange. I am baffled, but there's no way this is what it should sound like. I know that. It sounds really bad. It really feels like I'm trying to drive my phones from an ipod with the current power level I'm getting. Ideas please!? Gotta get this thing dialed in. Thanks
 
P.S tried all 3 sets of tubes and no difference in behavior

 

[tomb]

  Ok guys, just got to work and plugged in my dt-990 pro's. There is no way the sound I'm getting is how it's supposed to sound. I can very easily max out the volume pot and get to barely reasonable levels. Lots of distortion of too. No bass at all. Where shall I begin troubleshooting tonight? Am I correct in saying the output coupling caps are non polarized right? Just the stock Solen's, but there were no markings on them. I'm getting 272V at B+. And the choke is like a resistor correct? So no right or wrong way to put that in (ie backwards). All of my solder joints look real good. It seems like I have something in the wrong spot, or omitted something. All 4 Led's light up from the get go as soon as I turn it on, heaters on the tubes get nice and orange. I am baffled, but there's no way this is what it should sound like. I know that. It sounds really bad. It really feels like I'm trying to drive my phones from an ipod with the current power level I'm getting. Ideas please!? Gotta get this thing dialed in. Thanks
 
P.S tried all 3 sets of tubes and no difference in behavior

Give us some pics - that's the best way for us to check if you missed something.  Meanwhile,
 

1. Double check all of your resistors
2. Make certain that you did not install the TO-220 transistors backwards.  Some people have not paid attention to make sure the writing is facing out, not against the heat sink.
3. Make certain that you have the zener diodes installed correctly.  The black band should be facing the outside solder pad on both the left and right.

 
Carefully check these things.  The circuit is not really very complicated.

 

[tdubl07]

 
Give us some pics - that's the best way for us to check if you missed something.  Meanwhile,
 

1. Double check all of your resistors
2. Make certain that you did not install the TO-220 transistors backwards.  Some people have not paid attention to make sure the writing is facing out, not against the heat sink.
3. Make certain that you have the zener diodes installed correctly.  The black band should be facing the outside solder pad on both the left and right.

 
Carefully check these things.  The circuit is not really very complicated.

Just double check transistors, all good. I'll get picts up tonight. Thanks Hey Tom I'm rereading your post that says the zener diodes (d11,d12) sould have the black bands facing the outer solder pad. I thought Zeners the black band was the negative side. On both D11,and d12 the outer solder pads are positive on the pcb. If that's the case we have found the problem. I have the black band side soldered to the negative pads (the inner pad)

 

[tdubl07]

Well I think we've found the problem. I've got the zener's in backwards. DOH!!!!! Thanks Tom!!!

 

[tomb]

Just an FYI, but tdubl07 has also been communicating with me via e-mail.  It seems the Zeners were reversed, which would cause all of the symptoms reported.

 

[tdubl07]

  Just an FYI, but tdubl07 has also been communicating with me via e-mail.  It seems the Zeners were reversed, which would cause all of the symptoms reported.

Yep, I knew something was off, becuase man it didn't sound up to snuff. Thanks for the help Tom and everyone else, I will report back as soon as I get these installed properly. Hopefully I don't screw them up desoldering since the leads are mighty short as it stands now. A question though now that we have this figured out. Are the zeners actually installed "backwards" in this circuit since I thought black band was usually negative on zeners?

 

[tomb]

 

  Just an FYI, but tdubl07 has also been communicating with me via e-mail.  It seems the Zeners were reversed, which would cause all of the symptoms reported.

Yep, I knew something was off, becuase man it didn't sound up to snuff. Thanks for the help Tom and everyone else, I will report back as soon as I get these installed properly. Hopefully I don't screw them up desoldering since the leads are mighty short as it stands now. A question though now that we have this figured out. Are the zeners actually installed "backwards" in this circuit since I thought black band was usually negative on zeners?

Yes.  Dsavitsk explained it awhile back in this thread, but I forget the specific reason.  It's why I went to great pains to draw up the detail on the website.  I installed them backwards the first time, too.

 

[dsavitsk]

Are the zeners actually installed "backwards" in this circuit since I thought black band was usually negative on zeners?

Worth a quick read: http://en.wikipedia.org/wiki/Zener_diode

w/r/t/ power, ideally you might get 200mW from the amp. (The L-2 is more like 250mW.) I'd bet that in practical terms, it is more like half of that. That, however, is plenty of power.

People often misunderstand how much power they actually need to drive headphones. For instance, it is common to see claims that one needs a 10W amp for LCD2s. That 10W amp is usually rated that into 4 Ohms which means that it can swing about 6.5V. That's actually 800mW into the 50 Ohm load of the phones. Into 300 Ohm Sennheisers, it is only 130mW, meaning that this amp has as much power into 300 Ohm phones as a 10W power amp.

This is a major advantage of properly tapped output transformers in that they preserve power and adjust for voltage and current.

As noted, this amp is not going to drive orthodynamic phones.

 

[tdubl07]

Alright guys got the zener's fixed and this thing sounds awesome. Wish I would have seen the diagram before I put them in, but it was a good learning experience. Ended up soldering them under the board instead as it was far easier with all of the top populated with the heatsinks and wima's. This thing is absolutely dead quiet. No noise at all with no music playing. Just great. Here's a few pics, and I thought I'd throw a pic of the snubber caps installed as well just for reference if it hasn't been shown before, maybe help someone else in a new build visualize them. Thanks!

 

 

 

 

 

[tomb]

It's wonderful that you got it working - hope you enjoy!
 
Those are great pics!  I'll use the one for the snubbers, if you don't mind. 
 
It may take me awhile to properly post it on the website, though.  The US Post Office broke the shipping module on beezar again. 

 

 

[tdubl07]

Yes absolutely use that pic, that's why I put it up. Very glad to have got it sorted out. Fun build and sounds excellent. It will be right at home on my desk at work.