Rod Elliot 113 - Headphone amp TOO MUCH GAIN - diyAudio
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Old 29th May 2013, 11:29 AM   #1
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Default Rod Elliot 113 - Headphone amp TOO MUCH GAIN

I've built the Rod Elliot 113 headphone amp and although it sounds pretty good you only just need to move the pot (20k) before it's very loud.


I'm using the standard circuit except for the 20k Pot
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Headphone Amplifier

It looks like the gain is 23 (unless I've calculated wrongly), I'm thinking that reducing R4 to something like 4.7k to 6k would reduce the gain, I would appreciate any thoughts on if it's this simple or if anything else needs to change.

Setup is as follows:

Inputs: Pioneer 565 DVD, Sabre based DAC, PC (Xonar d2X)
Headphones: Beyerdynamic DT990 pro (250 ohm).

Cheers

Andy
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Old 29th May 2013, 11:51 AM   #2
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A gain of 23 is way too much.

On my own discrete headphone designs I use a gain of 2.5, which is enough for 5VRMS output if you use a 2VRMS source. That gives me 100mW into my 250 Ohm DT880 PRO, which is enough for 116db peaks, more than sufficient, unless you like getting hearing damage.

The problem with low gain settings, are that unless the headphone amp has been desgined with that in mind from the beginning, you could end up with stability issues.

However, since this design is a composite amp, with a simple discrete buffer within the feedback loop of an Opamp, I cant really give you a definitive answer, you might just have to try and see if it works.

Last edited by Neutrality; 29th May 2013 at 11:57 AM.
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Old 29th May 2013, 11:57 AM   #3
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Hi Neutrality,

Thanks for the post. I recon my inputs will vary between 1.5 to 2 volts so your gain sounds good for me.

I see your point regarding stability at lower gains, I should have mentioned that I'm using an LM4562 op amp for the voltage gain, which I think should be good for low/no gain.
I don't know much about the transistor buffer stage though and how this is affected, I would be grateful for any opinions on this.

Cheers,
Andy
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Old 29th May 2013, 12:03 PM   #4
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According to the datasheet, the LM4562 is Unity gain stable so 2.5 gain should not be an issue.
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Old 29th May 2013, 06:16 PM   #5
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Default Rod Elliot 113 - Headphone amp TOO MUCH GAIN

I have built the P113 using the LM4562 with R4 set to 3k9. If you want to go lower than this bear in mind that R5 & R6 also present a load to the opamp, so it might be better to increase R3 to 1k5 or 2k2 if you need less gain.

I used a 10k pot but changed R1 to 47k. Ideally, R1 should be substantially higher than the pot value. I also omitted R2 completely. This now results in "unbalanced" input currents so you should ideally use a FET op amp like the OPA2132 or OPA2107, but I found the LM4562 having low input bias current and offset voltages for a bipolar works fine. It also gives slightly more sparkle to the sound than the OPAs. However, the difference is only slight. Nowhere near the difference you would expect in a CMOY design.

A few other points that might help. Different opamps may not make a huge difference but the quality of the pot does. Don't use a cheap carbon pot - it sounds horrible. C2 should be a bipolar electrolytic. Bypass them with a film cap. I just had a few 1uf Wima polyesters but they gave an increase in treble definition.

Rod recommends 120 ohm output resistor but this makes the output impedance quite high. I used a 4r7 resistor instead.

Hope this helps.
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Old 29th May 2013, 07:42 PM   #6
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Richard,
Thanks for all the info. Before I saw your post I changed r4 to 2.2k and it was a bit hairy with the PCB pads so I don't think I dare risk swapping again. I will get a better 10k pot as the one in using is an akps 20k from a goody bag. Re the bipolar cap in using a polarised electro and it seems ok. I have some 2.2uf wima film caps would these be ok for bypass? Do you know what this bi cap does and also how bypassing helps?
Thanks again for the info.
Andy
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Old 29th May 2013, 08:15 PM   #7
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Andy

Polarised electrolytics work best with a reasonable level of DC voltage across them. In this circuit we are only talking about millivolts. Rod Elliott in his application note (you can access this if you bought the PCB from him - he will have sent you the password) presumably changed the cap to a bipolar because of this.

However, if it works fine don't change it - no damage will result. Bypassing an electrolytic with a film cap sometimes yields an improvement in audio quality - depending on the circuit. Electrolytics are technically the worst kind of capacitor but are unavoidable for large values.

In this case there was an improvement, not spectacular but nonetheless an improvement. This is with Sennheiser HD650 headphones. You should be able to solder the Wima caps you have to the underside of the PCB in parallel with the electros. Give it a try. No harm will result by leaving them in place even if you think they have not made things better.
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Old 30th May 2013, 03:10 PM   #8
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Just finished my build of p113, what a fantastic headphone amp. I used a 100k ALPS pot (like the op, all I had handy), and two opa627 on an adapter. Even without burning it in, the soundstage is excellent.

I'm using DT880 (600 ohms impedance) without any volume control problems.

What is the technical reason a 100k pot is inappropriate here?
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Old 30th May 2013, 05:01 PM   #9
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Quote:
Originally Posted by lordearl View Post
What is the technical reason a 100k pot is inappropriate here?
Noise.

Sorry for my non-technical response; I'll leave it to someone else to elaborate.
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Old 30th May 2013, 09:57 PM   #10
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Exactly, and depending on opamp type and its common-mode distortion performance, linearity with increased source impedance doesn't exactly improve either. With a 100k, source impedance can go as high as 25k, which is a bit much. Incidentally, voltage noise on the OPA627 is equivalent to <2 kOhms, so noise would actually be limited by the pot at some settings even with a 10k type.

This amplifier pretty much is a plain vanilla opamp + AB buffer design (plus bias bypass). It should perform well. A few notes though:
1. As mentioned, the stock gain of 27 dB is excessive. With a weak source (~1 Vrms) and K240Ms or the like, I might see R4 values of up to 8k2 or 9k1 making sense (i.e. up to 20 dB), otherwise you'll probably be best off in the 2k2...3k9 range (10..14 dB). The buffer should have a bandwidth of 50+ MHz, so it would take a very fast opamp (by audio standards) to run into trouble even at unity gain.
2. R2 may still be good for keeping RF out, but as shown, the impedance seen by the +input is always greater than the one seen by the -input at any volume setting. Drop it below R3||R4, and you'll see them equal at some low volume at least.
3. According to simulation, you can replace C3-C4 with just one cap (connected from C3+ to C4-). Seems counterintuitive, but it appears to be totally equivalent.

Last edited by sgrossklass; 30th May 2013 at 10:01 PM.
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