• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Output cap for OTL headphone amp

I've used Elna Silmic RFS and Nichicon FG 'Fine Gold' electrolytics to good effect as output caps;

Elna Silmic RFS Electrolytics | Hifi Collective

Nichicon FG Type | Hifi Collective

I'm currently working on a HPA project and have a pair of the Nichicons installed for the output caps for 32ohm headphones, however, most of my headphone listening is with 600ohm BeyerDynamic DT880 heaphones and that impedance brings film output caps into range (I'm using 56uF film caps)

Of course these are just my own views on the suitability of those caps and YMMV.
 
I've used Elna Silmic RFS and Nichicon FG 'Fine Gold' electrolytics to good effect as output caps;

Elna Silmic RFS Electrolytics | Hifi Collective

Nichicon FG Type | Hifi Collective

I'm currently working on a HPA project and have a pair of the Nichicons installed for the output caps for 32ohm headphones, however, most of my headphone listening is with 600ohm BeyerDynamic DT880 heaphones and that impedance brings film output caps into range (I'm using 56uF film caps)

Of course these are just my own views on the suitability of those caps and YMMV.

Those won't work for the Morgan Jones amp as it runs at 220V DC.
 

edwtsui

Member
2017-10-20 6:58 am
Why should you? There are plenty of suitable caps with higher voltage rating, such as these 400MXC220MEFCSN30X30 Rubycon | Capacitors | DigiKey

Which is what I used when I built the MJ 14 years ago, //ed with some nice film caps.

It is because those audio grade electrolytic caps are limited to 100VDC, that's why I am thinking to lower the DC offset on the output. Is the Rubycon good for audio?
 
Rubycon caps could be considered audio grade... it is highly subjective. What makes an electrolytic cap 'audio'? It is still a electrolytic cap,
and I'd rather use one with the appropriate voltage rating (with a safe margin) than to alter the B+ of the amplifier (btw. this is not DC offset!!
100V of DC offset will absolutely positively destroy any headphone) which will likely have an effect on the sound of the MJ.
 

Tjj226

Member
2012-12-02 2:49 am
Just as an FYI, the value of the output capacitor is determined by the load resistance. 470uf is a good value for 32 ohm headphones, but if you have a pair of higher impedance headphones, this value can be a lot lower. For instance 60uf would be a decent value for headphones like HD600s.
 
Any cap will do, nick one out of an ATX PC PSU or other SMPSU, but usually one made by Nichicon/Rubycon, Sony , Panasonic etc is a good bet, though it helps if you choose one with a nice paint job, something black with Gold writing : )

Actually thinking about it I'd presume you'd want one with low ESR but as most 470u 200v ish caps are made for use in SMPSU's they should be good at higher frequencies.

A look in Digikey and Mouser should give you some candidates though.

Andy.
 

edwtsui

Member
2017-10-20 6:58 am
Hi,

I have another question regarding to the PSU design. There is a 220uF cap placed just next to the rectifier, would it be too large?
 

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Hi,

I am going to build the Morgan Jones headphone amp. My challenge is that it requires a 470uF cap for output. The cap is so large such that a film cap is not possible and electrolytic is the only choice. Could anyone suggest a good electrolytic for me?


https://www.pearl-hifi.com/06_Lit_A...04/Sec_19/991_White_Follower_Optimization.pdf

I calculate 26uf is fine if you can use 300 ohm headphones. I just happen to be bread-boarding a 6AS7 OTL headphone amp this week and just rationalized myself over this same issue!

C = 1,000,000 / (f * 2 * pi * R)

so for -3db down at 20Hz with 300 ohm phones.

C = 1,000,000 / (20 * 2 * 3.14 * 300)
C = 26uF

In that range you can use a polypropylene film capacitor instead of an electrolytic:

I'm waiting for delivery on this 30uF to try out, with Sennheiser 650 phones (300 ohms). 30uF will cutoof a little below 20 Hz I assume. The 450 volt AC rating should be fine for DC which would (I think be 1.414 * 450) even better.

CBB60 AC 450V 50/60Hz 30uF 8mm Thread Polypropylene Film Motor Run CapacitorZ7U5 191466582282 | eBay
 

Tjj226

Member
2012-12-02 2:49 am
I calculate 26uf is fine if you can use 300 ohm headphones. I just happen to be bread-boarding a 6AS7 OTL headphone amp this week and just rationalized myself over this same issue!

C = 1,000,000 / (f * 2 * pi * R)

so for -3db down at 20Hz with 300 ohm phones.

C = 1,000,000 / (20 * 2 * 3.14 * 300)
C = 26uF

In that range you can use a polypropylene film capacitor instead of an electrolytic:

I'm waiting for delivery on this 30uF to try out, with Sennheiser 650 phones (300 ohms). 30uF will cutoof a little below 20 Hz I assume. The 450 volt AC rating should be fine for DC which would (I think be 1.414 * 450) even better.

CBB60 AC 450V 50/60Hz 30uF 8mm Thread Polypropylene Film Motor Run CapacitorZ7U5 191466582282 | eBay

With headphones you can actually hear down lower than you can with speakers. Ideally you would want to be flat at 20hz with little to no phase distortion. I found that 60uf does a really great job with 300ohm headphones.