OTL headphone amp distorting & clipping: weak power supply? - diyAudio
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Old 20th March 2014, 10:42 PM   #1
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Default OTL headphone amp distorting & clipping: weak power supply?

I'm new to the boards and had a question about power supplies, but am not sure if my question should go here or in the Headphones section.

I have a 10+ year old Antique Sound Lab headphone amp that was a prototype of which only about 8 were made: the MG-Head OTL-H. I purchased it off of the Head-fi boards years ago and have had one lingering problem with it: it clips and distorts when the volume is turned up about halfway. There aren't any output transformers and the owner of ASL admitted that it has a weak power supply. Here is some background reading on what I'm dealing with.

A toast to Joe Lau!!!
Help me fix MG Head distortion!

Now, I have measured the voltage at Pin 3 on each 6BQ5 and it's reading about 125VDC. There also seems to be a JFET diode cascode CCS tacked in there, supplying power to the anodes. It's tough to tell what's going on as I can't really get the board out.

How can I improve or rebuild the power supply to provide adequate power to this 6BQ5 OTL design so that there isn't any more clipping?

Thanks in advance,
Dan
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Old 21st March 2014, 09:52 PM   #2
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After more investigating, this amp seems more and more strange.

I have discovered that there are 2 DN2540 MOSFETs inside this amp. There is one DN2540 for each EL84, going to pins 7 and 9. The Gate and Source pins on each unit are strapped together with a 47Ohm resistor, then going out to the plate. I'm not very familiar with this type of architecture. Is this a half-hearted or just a simple CCS?

Would a cascode CCS like the K&K kit work better in this instance?

photo.JPG
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Old 21st March 2014, 11:25 PM   #3
00940 is offline 00940  Belgium
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You mean like this ? It's simply a triode strapped EL84 with a simple (but ok) ccs.

Is the output to the headphones jack taken in between the resistor and the mosfet ?
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Old 21st March 2014, 11:49 PM   #4
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That's exactly what's going on. There is a slight variation between each CCS that seems to be causing a channel imbalance. If I replaced both with a single cascode CCS, giving me the same B+ for both EL84s, would that give me a more balanced sounding output?

On a different note, what about replacing my whole power supply and keeping a CCS to get a higher B+ on the plate? Right now it's at about 120VDC. Would bringing it up to around 160 help get rid of the distortion?
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Old 22nd March 2014, 12:55 AM   #5
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Quote:
Is the output to the headphones jack taken in between the resistor and the mosfet ?
Yes- They are all tied together at the same point for each output. Sorry, I'm very new to trying to understand what's going on with tube design. Thanks for helping.
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Old 22nd March 2014, 01:28 AM   #6
00940 is offline 00940  Belgium
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The channel imbalance is probably due to the tubes more than the ccs.

It would help to know more... What is the role of the input tube ? Is there some sort of global feedback ? What's the total gain ? What headphones are you using it with ? What is the voltage of B+ now ?

Quote:
Originally Posted by dludingt View Post
Yes- They are all tied together at the same point for each output. Sorry, I'm very new to trying to understand what's going on with tube design. Thanks for helping.
Then, the mosfet aren't really used as ccs. It's more an hybrid -follower, to get a decently low output impedance.
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Last edited by 00940; 22nd March 2014 at 01:54 AM.
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Old 25th March 2014, 01:17 PM   #7
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00940,

Thanks for the advice so far. Here is what I know about this amp and how I'm using it. I have attached the schematic of the headphone amp that my unit is based on. The only apparent difference I can tell is that the output transformers have been replaced by this MOSFET board with the DN2540's.

I know that the channel imbalance is not due to the tubes, as I have swapped out the EL84's and the 12ax7 to multiple variations with the same result. I have 4 or 5 pair of EL84's and 5 or 6 driver tubes that I have tried.

1. The role of the input tube seems to boost gain. I have tried numerous 12ax7 and 5751 tubes in this position.

2. I'm not sure what the total gain is for the design. How would I determine this?

3. I'm using Sennheiser HD-580's with this, so their impedance is 300 ohm.

4. The voltage of the B+ is 164.4 VDC.

Since I'm learning about all of this, how should I proceed? I'm trying to figure out how to simulate this in SPICE, but I'm still climbing up the learning curve.

Thanks in advance for any help.
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Old 25th March 2014, 03:37 PM   #8
00940 is offline 00940  Belgium
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Hi,

if I'm not mistaken on some values, going from the picture and the schematic, here's what you've got (trying to respect the names from the schematic):

Click the image to open in full size.

I calculated a new value for the el84 cathode resistor (r11). This gives about 125V on the tube's plate, as measured.

With a 12ax7 at the input and no-feedback (feedback resistor is missing), gain is crazy high. With a 0.1Vrms input (after the pot), we're already at 5Vrms. I'm surprised you can even turn the pot halfway (the HD580 are very loud at 2Vrms).

Output impedance is also huge (I mean, since the cathode resistor is apparently unbypassed, we're at 3K).

Still... sims show it should work without crazy distortion up to 2Vrms (1% thd point). I assume the 164V B+ is what you get from the external supply ? Is it a measure under load ? Consumption being constant (due to the CCS), weak supply or not, it shouldn't make a difference. If it's unloaded, then we might have a problem.
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Old 25th March 2014, 03:42 PM   #9
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Ben,

I made the B+ measurements from the external supply when it's unloaded. And you're right with there being about 125V on the plate.

Also, you're right about the distortion being when the volume gets to almost unbearable levels.

So, where do I go from here? Why is the 164V B+ a problem unloaded?
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Old 25th March 2014, 03:51 PM   #10
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I just measured the B+ under load and it's about 126V.
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