mwl168

Sharing my findings with attempts to raise output BJT current as promised. I am running +/- 16V rails. Using 255R in R38 and R39, I got around 186mv voltage drop on the output resistors (20R) when the amp is fully warmed up (160mV at cold start). That's around 9mA current. I could barely feel a temprature on the output transistors at this operating condition. I finally settled on 301R in R38 and R39 which results in 250mV across the output resistors at cold start, rising to and stablize at 320mV after a few minutes. The output BJTs are quite warm to the touch but not hot. I am really liking the sound of the amp at this condition. For what it's worth, here is a few other values of R38 and R39 I tried and the resulting voltage drop across the output resistors: R38/R39 = 287R, 230mV cold start, 273mV warmed up R38/R39 = 317R, 300mV cold start, 360mV warmed up R38/R39 = 402R, 500mV cold start, did not wait for it to warm up. This amp really sounds great on both my Senn. HD650 and LCD2. Subjectively the best among the headphone amps I own and heard. Many thanks Kevin for your ingenuity and help!

Thanks Kevin. I'll experiment with increasing the current of the output section and share my findings here.

Thanks Kevin. That worked. So what does the pot next to the LEDs do? What about my first question? Should I or can I increase the value of the bias resistors (R38, R39) to increase the current across the 20R emitter Resistors? How much current is idea for these BJTs to operate in? I built this amp primarily for my LCD2 so, I think, higher output current is probably more important than high voltage swing. For the benefit of others that might have the same question, I found those two center pots interact with each other some so I had to adjust both back and forth so both sides gradually converge.

I finished building my SuSy Dynalo today and it's making music now. It appears to be working fine; no buzz no hum, dead silent with no input signal and drive both my headphones with gusto. But I have two possible issues. The amp runs on +/- 16v, 255R bias resistors (R38, R39). I hand-measured all resistors and matched all LEDs and BJTs (mpsw06 and 56) to within 0.1% using the circuits Kevin showed in his original Dynalo article. I populated all parts but did not install the server OPs. All trim pots are adjusted to its center position. Upon power on the first time, one board shows DC offset of 7mv and the other 5mv (measured between O+ and O-) and are very stable. The DC offset between either phase of out to ground is around 60mv for one board and around 45mv for the other. Here are my questions: 1. The voltage drop on the 20R resistor is only around 185mv after 60 minutes running. That's less than 10mA, much lower than that of the Dynalo (around 15 mA). Is this normal? The mpsw06/56 are rated at 1W dissipation. This setting seems extremely low. 2. I tried turning the trim pots by the LEDs to adjust the DC offset and they seem to have no effect at all - three full turns one way or the other does not seem to do anything to the DC between O+ and O-. What gives? Otherwise, the amp seems to be working fine. Can someone shed some lights? Thanks!

The PCB boards I ordered from SeeedStudio came in today. These are the latest Rev 1.02 boards. 1.6mm thickness, 2oz copper. I ordered a few extras. PM me if interested.

Thanks for the response Pars. Edit: Pars was right. I was not thinking straight. If + and - outs are adjust to 0 in reference to ground, the offset between + and - will be 0 naturally. In my experience, the offset will have some thermal drift during use but since both phase outputs drift in the same direction, the offset between + and - out is very stable.

Thanks Kevin. I assume you meant getting the + and - output to zero in reference to the ground on each channel. Do I need to concern about that if I only intend to use the balanced out? It's been a while so I may not remember correctly, but I seem to recall observing this phenomenon when trying to build the original balanced bridge Dynalo - I could get the DC offset between the + and - to 0mv, but there would be significant DC offset from either phase to the ground and they were not symmetrical.

Are the servos aboslutely necessary? If the parts are well-matched, is it possible and safe to leave out the servos and simply use RV3 (in the production schema published here) to null out the output DC offset? I've used similar practice in my single-end Dynalo build which uses hand-matched parts including 0.1% resistors and did not have problem with DC offset. Does the balanced topology of the Dynalo MK II make the servos more ciritcal?

I am planning a SUSY Dynalo build and came across this 4-deck stepped, 10K attenuator on eBay from Taiwan. It uses SMD resistors but is only 23 steps. I posted the link below in case anyone is interested in looking into it. http://www.ebay.com/itm/Balance-XLR-23-Stepped-Attenuator-Potentiometer-10K-Log-/301415257011?pt=LH_DefaultDomain_0&hash=item462dbfd3b3

Yes, I am looking for the SS Dynalo PCBs. Are your boards the final production version? I do not need PSU, will likely consider using the Twisted Pear Audio Placid BP. Thanks!

Does anyone have a pair of the production Multiamp PCBs for sale? I am also interested in a pair of the SMT step attenuator PCBs as well. Thanks!