Rod Elliot 113 - Headphone amp TOO MUCH GAIN - Page 2 - diyAudio
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Old 31st May 2013, 07:44 AM   #11
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Thanks for raising these great points.
Generally my favourite opamps (on dual adapters in this case) are AD825 and OPA627, however I notice that OPA627 in this circuit gives the best bass clarity (thought the sound is noticeably 'darker'). I'm not familiar with voltage noise measurements - are you suggesting that the noise is limited by the pot, no matter what value pot, hence the 100k pot should be ok in this instance? I certainly don't hear any noise, quiet is a grave!
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Old 31st May 2013, 05:12 PM   #12
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Quote:
Originally Posted by lordearl View Post
I'm not familiar with voltage noise measurements - are you suggesting that the noise is limited by the pot, no matter what value pot, hence the 100k pot should be ok in this instance?
No, certainly not. Depending on volume setting, the 100k pot would give up to 10 dB more noise than a 10k, the effect being greatest around the -6 dB point. Actually it wouldn't become as bad as this here, as the parallel R2 would limit maximum source impedance to ~12.7k (and distort the pot characteristic near the upper range). So the noise penalty over a 10k would probably be more like 5 dB in this circuit. Normally R2 should be > Rpot.

Total input-referred noise is probably best calculated like this:
1. Determine impedance seen by noninverting input.
2. Ditto for inverting input.
3. Add the two.
4. Determine equivalent input noise density en = sqrt(4 k T R) ~= sqrt (1.63E-20 * R) @room temp
5. RMS sum resistor en and specified opamp en, i.e. en,tot = sqrt(en,R + en,OP)
6. Total noise Vn = en * sqrt(bandwidth) ~= en * 141 @20 kHz

Actually I created a little spreadsheet last year that does this sort of stuff and conveniently calculates output noise and SNR. There you go. It's an OpenDocument thingy (Open/LibreOffice, M$ should have converters nowadays). I don't have an R2 equivalent in there, but it should be useful as-is.
Quote:
Originally Posted by lordearl View Post
I certainly don't hear any noise, quiet is a grave!
That's not terribly surprising. DT880-600s aren't all that sensitive, around 96 dB SPL / 1 V. They'd probably still keep noise generators like a TDA2822 in check, or your average integrated amp for that matter. Designs like P113 were basically created for headphones like these.

Your amp now has a worst-case noise level of around 50 V @20 kHz BW. That's like 10 dB SPL with these cans. At normal volumes it would probably be even less, about 4-5 dB SPL. The threshold of audibility is in the 20-30 dB SPL vicinity, depending on headphone type. IOW, this combo should be as dead silent in theory as you found it to be in practice.

You should also be around the -30 dB point in terms of volume setting, which is a totally unproblematic range for most any pot.

Now enter some headphones or in-ears which are 30 dB more sensitive (they're not uncommon), and it's a totally different world. Noise becomes distinctly audible, and operating a volume pot around -60 dB usually is not much fun even if it does keep decent channel tracking. Hence, this thread.
(Though I'm a little surprised that the OP would have run into trouble with DT990s, which aren't that much more sensitive.)

Last edited by sgrossklass; 31st May 2013 at 05:28 PM.
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Old 31st May 2013, 09:43 PM   #13
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Sgrossklass,
I just wanted to confirm the trouble I had with the DT990's wasn't to do with unwanted noise, hiss etc. It was to do with the standard gain of the circuit design which was huge so even with not sensitive beyers I only had to move the pot (20k ohm) by a few degrees from minimum before my ears were blown off .

Cheers,
Andy
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Old 1st June 2013, 02:00 PM   #14
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I find the stock gain perfect for my non-over sampling ad1865 dac with passive I/V......though the output is about 125mVrms so I can certainly see how the p113 would have too much gain for a source with 2vrms.
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Old 1st June 2013, 07:38 PM   #15
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That's 125 mVrms fullscale? That would be 24 dB under CD player level. So with a standard CDP you'd be getting along just fine with a whopping 3 dB gain in the headphone amp, in spite of the rather insensitive DT880-600s. That would seem to fit; even with rather insensitive 600 ohm cans (HD420SL) my home listening levels tend to be at a few dozen mV at best, and I tend to be setting my Clip+ for about 80 mVrms FS with typical ReplayGained (89 dB SPL / -14 dBFS avg) non-Classical recordings.

That would already put us at -55 dB with a CD source and stock P113. Which happens to be about the range where people start complaining about touchy volume setting. And that's with insensitive cans, albeit at lowish levels. Oops. (I'm not sure how I got to -30 dB earlier.)

It's certainly interesting. Stock P113 gain is sized for an input sensitivity of about 400 mVrms (minimum level required for maximum output). That already is 6 to 8.5 dB higher than on many integrated amps, yet still makes for a touchy volume pot. (Complaints about touchy volume or bad tracking are not uncommon on integrated amps, as these are dimensioned like they were 20+ years ago but average levels on CDs have gone up by 10+ dB.) Then again, stock P113 could drive DT880-600s to about 115 dB, while your average 100wpc speaker amp will get maybe 105 dB out of average sensitivity speakers. The difference in input sensitivity is not enough to offset this, the headphone combo still remains somewhat more sensitive. With more sensitive cans, it gets even worse.

Still, it might not hurt to double check the volume pot for goofs - like miswiring or wrong type (e.g. linear instead of log). Murphy never sleeps.

Last edited by sgrossklass; 1st June 2013 at 07:51 PM.
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Old 3rd June 2013, 02:25 AM   #16
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Exactly (almost) right – in fact, the AD1865 used with 200R i/v resistors (unbalanced) gives 127mVrms! I’m using 91R MK132 resistors so the output is probably less than 100mVrms. The P113 is almost acting like a higher current i/v gain stage, and gives listening volumes around 12PM on the volume pot. Works very well indeed!

I once had an NAD integrated amplifier, with a cd player as the source the volume couldn’t go past 9PM without being too loud, but the channel tracking was absolutely terrible!
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Old 9th June 2013, 02:15 AM   #17
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Thought I'd hook my p113 up to a standard 2vrms cd player to see what other users are experiencing and oh boy, the gain is FAR too high, even for my 600 ohm headphones. The pot also gets a lot noisier, to the point of being annoying during silent passages in music.

I also read on rod elliott's page that this amp could be used to power efficient speakers. So, I took the volume pot out & attached it to some 90dB floor standing speakers I have and lo and behold, with a low gain preamp and standard 2vrms source, we have a beautiful 5W power amp! In fact, the sound with speakers easily rivals my 6V6 push pull tube amp AND uses 5% of the energy!

Presumably the 120r resistors at the output are not needed if we are using this circuit with 8 ohm speakers?
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Old 9th June 2013, 06:18 PM   #18
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Quote:
Originally Posted by lordearl View Post
I also read on rod elliott's page that this amp could be used to power efficient speakers. So, I took the volume pot out & attached it to some 90dB floor standing speakers I have and lo and behold, with a low gain preamp and standard 2vrms source, we have a beautiful 5W power amp!
Actually, he himself writes,
Quote:
The amplifier is capable of delivering around 1.5W into 8 ohm headphones, and 2.2W into 32 ohms
Which seems about right for medium power transistors like those used. (That's about 600 mA peak at full power into 8 ohms.) Earle Eaton used beefier types in his amp.

Ultimately there's only so much an amp with a single-stage buffer can drive though. A 5532 can supply up to 25 mArms or so, and you get a beta of maybe 50 with good low-droop transistors. That would still be good for ~10 W into 8 ohms (voltage limited at +/-15 V) or 6 W into 4 ohms, but your average LM317/337 or 7815/7915 supply would collapse first and require some beefing up.

Ultimately you'd be looking at a two-stage (e.g. EF2) buffer and a matching power supply, possibly some Zobel network at the output as well. Still, an opamp-based concept makes a fair bit of sense for a low-power amp. You could get maybe 20-25 watts into 4 ohms with common opamps, which is a workable amount for sure.

This amp uses an opamp with CFP output, though I must say I don't like the biasing with only one diode. it's either underbiased or thermally unstable.
Quote:
Originally Posted by lordearl View Post
Presumably the 120r resistors at the output are not needed if we are using this circuit with 8 ohm speakers?
No, unless you're happy with a 40 mW / 8 Ohm, current output amp that is.
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Old 15th June 2013, 04:12 PM   #19
paulb is offline paulb  Canada
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Quote:
Originally Posted by redal View Post
... I only had to move the pot (20k ohm) by a few degrees from minimum before my ears were blown off
Are you sure you're not using a linear taper potentiometer instead of a logarithmic taper? Rod has an article on his site about how to fake a log taper from a linear pot.
If you're not sure, set the pot at its halfway position and measure resistances from the wiper to the other two points. They should vary by maybe 10:1 for a log pot.
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