R124 and R125 are load resistors that help to drain the output coupling capacitor so that it doesn't hold a high voltage that might cause a pop when you plug phones in. Since this amp can drive 30ohm loads just fine, and your cans are 300ohms, having say, 300ohm or 270ohm R124/125 would be equivalent of a 150ohm load on the amp, so that is not a problem. I would use anything from 270ohms to 680ohms 1/4w or 1/2w if you have it.
The HD560, Btw, is the *Ideal* headphone for this amp. You will love it.
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Balanced Drive SE Class A HPA
In case anyone has a DAC or preamp with balanced outputs (+ve and -ve phase balanced line driver), and they have headphones modified to accept balanced drive amps, there is an easy way to make yourself a unique, and very cost effective balanced SE Class A HPA. Just use two DCA boards, one for each earpiece (driver). By doing this, you get 4x the power, and twice the slew rate. Best of all, because this is a SE Class A amp that is cap-coupled, you never need to worry about the amp going DC on you and blowing out your expensive cans. I have tried this with the PCA before and it sounds fantastic. So much dynamic reserve, and everything sounds so much more powerful.
Here is schematic:
So, if you were to run each amp with Vcc of 25v, and 150mA bias current, with 42ohm cans you can get about 2.2wrms and +/-20vpp swing from an effective Vcc of 50v. This is useful for super inefficient 74dB planar headphones.
In case anyone has a DAC or preamp with balanced outputs (+ve and -ve phase balanced line driver), and they have headphones modified to accept balanced drive amps, there is an easy way to make yourself a unique, and very cost effective balanced SE Class A HPA. Just use two DCA boards, one for each earpiece (driver). By doing this, you get 4x the power, and twice the slew rate. Best of all, because this is a SE Class A amp that is cap-coupled, you never need to worry about the amp going DC on you and blowing out your expensive cans. I have tried this with the PCA before and it sounds fantastic. So much dynamic reserve, and everything sounds so much more powerful.
Here is schematic:
So, if you were to run each amp with Vcc of 25v, and 150mA bias current, with 42ohm cans you can get about 2.2wrms and +/-20vpp swing from an effective Vcc of 50v. This is useful for super inefficient 74dB planar headphones.
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I calculate that 470uF output is good enough for my 32 Ohm phones.
This gives a roughly 10Hz lower frequency.
Of course anything more, like 2200uF, will not hurt.
Especially if you have 8 Ohm phones you need 2200uF.
470uF will give you around -3dB at 10Hz. Electrolytic caps will add distortion at this frequency, making the cap a bigger value will lower distortion. There's also the matter of LF phase shift which is increased with smaller valued caps. I myself prefer to use at least 2200uF with my 56ohm Superluxes.
That was fast for first class parcel shipment via USPS. How are you doing on parts to make it?
I think I got a cart at mouser ready but I did it late at last night so will have to check over it before hitting buy..didn't expect the boards for another week haha
I added mostly yaego resistors as vishay were mostly not available in the cmf of the bom
I've also been mentally trying to work out how to securely attach the switchcraft sockets to the board
I added mostly yaego resistors as vishay were mostly not available in the cmf of the bom
I've also been mentally trying to work out how to securely attach the switchcraft sockets to the board
I thought you had one already? Well, I can't give you an entire Aksa Lender preamp for free, but you have spare TH boards now to make an HPA if you ever order the main board and set of DB's.
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If you have a linear supply that will be fine, 24v will end up around 20v after the cap Mx and CRCRC. It will work fine but I think a DC step up may be quieter depending on how quiet your linear supply is. I prefer a linear class 2 transformer to drive the DC step up. That is quieter than most SMPS driving a DC stepup. You can also just use an LM317 linear regulator to drop say 27v to 24v and let you adjust voltage.