[xrk971]

I am going to be at the Schiit Storm this Saturday in Gaithersburg. Will be bringing Pocket Amps, Silicon Harmony, and HyQu for demo.

See you there!

https://www.head-fi.org/threads/dc-...-august-12-2017-gaithersburg-maryland.855135/

 

[gwompki]

@xrk971 I know you mentioned having a set of KZ ZS5. I got them recently as well and was wondering what cap configuration you were using in the PCA with these. The ZS5 seems to be lacking a bit of dynamics with the amp in stock form. Guessing that is due to their very low impedence.

 

[stellarelephant]

For driving very low impedance I think you'll want the largest 16V output caps possible, and you might want to tune bias a little higher.

Here is my 63mA build, newly outfitted for passive cooling. I painted the inside of the lid for heat transfer. Thanks for the link to the tiny heat sinks, Funch!

 

[xrk971]

Gwompki,
I am currently using 1000uF Panasonic FR's and they sound fine relative to dynamics. I need to build a new one with dual 1000uF OSCONs and see if better. In testing, they almost as powerful as full a size headphone on a desktop amp. I can't have my iPhone volume above 70% with PCA on full volume playing some jazz with drums. The drums are very dynamic. The highs are superb with a super tweeter actually sticking into the ear tube. Did you get the upgraded gold cable? That's what I am using - have not even bothered to use stock black vinyl cables.

 

[gwompki]

Thanks XRK and stellarelephant. I do have a set of 1000uF panasonic FR that I will try to swap in and see. I will also try tuning the bias a bit to see what that does. Looking forward to trying them out!

I did not get the upgraded cable for the ZS5 as I heard the only benefit was less microphonics. If you think its a worthy upgrade though I will give it a shot eventually.

 

[stellarelephant]

Gwompki,
Another thought on maximizing performance with those 16 ohm IEMs... I know you were using the EBL batteries. If you want to quickly test the effects of increasing bias before changing resistors, swap in some fresh alkaline batteries and see if you can hear a difference. 18v vs 16.5v.

 

[xrk971]

Good idea StellarE! I like simple things that you can try right away with minimum cost/effort. Actually, I always ship my amps preloaded with a set of alkalines to let the customer enjoy right away and to verify that it works. Most people don't have rechargeables on hand.

 

[gwompki]

Alright think I discovered the reason for the big discrepancy in the sound between my new build and my first one. I have swapped out quite a few caps on the output now and I think what it came down to is the whole time was I had the polarity backwards when using electrolytics. Doh! The low end has returned to my new build and with the 1000uF caps it is quite a fire breather.

Time for even more tweaking!

 

[xrk971]

Glad you found the cause. The caps become a dead short when backwards and you are lucky it did not blow up.

 

[gwompki]

:O! That's terrible...I had been listening quite a bit that way. I'm not 100% sure I had them backwards, but I kept feeling the low end was lacking and when I put the latest round in the bass definitely returned in a big way.

 

[stellarelephant]

Quick update: my mosfets now stay nice and cool with the tiny heat sinks and painted tin interior (which defeats the radiant barrier nicely, dissipating most of the heat right through the lid). I can run the amp indefinitely and the sinks become just mildly warm to the touch.

 

[sdouble]

Hi guys , as you may know, i'm a noob in the audio business. so please forgive me if my question is idiotic. Looking at the amp scheme I wonder why you put the pot in the input stage. A pot is a noisy device. In principle, I guessed that it may be worthwhile to put the pot after the BF862 amplification stage.

 

[xrk971]

A pot is same as two resistors acting as a voltage divider. Once the pot is stationary, the noise is Johnson noise from two resistors - not much. Should we avoid resistors in our circuits because it adds noise? Well, this amp sort of does that with a minimal number of resistors but that wasn't the point. Too many people worry about pots in the signal path - use a decent moderately priced (under $5) one like this Alps RK09 series and it works fine. This amp has no scratchy noise when adjusting volume because there is no current through the high impedance gate of the JFET. If you put the pot between the JFET and the MOSFET, there is more current there because the capacitance of the MOSFET gate is higher, and there will be more noise especially when adjusting volume while music is playing.

 

[sdouble]

my pint was that the pot is not just a resistor. The trimmer may grab some noise.
I agree that the input cap of the mosfet is much higher than that of the JFET but the imput impedance of the MOSFET is also high. I intend to use a SMD pot, the Bourns PTR902-2020K-A103-ND

 

[stellarelephant]

Woot woot! DIYAudio member Raptorlightning has successfully implemented a USB-charging single cell lithium battery pack in the Pocket Class A! The amp can also run directly off USB power with this setup--a major benefit IMO.

He used a LiPo cell and three small circuits: an off-the-shelf boost converter board, a custom load-sharing board with a micro USB power input, and a tiny custom cap multiplier board that keeps the battery from going into auto-protect when the big capacitors charge up at turn on.

I'm hoping to do the same for my builds as soon as I fully understand his approach.