KG Balanced Dynahi build discussion thread

[jwzhan]

72VCT@2A per channel. Better to use two PSUs if you intend to push the amp board to the limit, so you will need two separate windings in that case.

[Valentin Hogea]

72VCT@2A per channel. Better to use two PSUs if you intend to push the amp board to the limit, so you will need two separate windings in that case.

 

Don't have that much knowledge on transformers. But I have these specs for the KGSSHV.

 

 

450VAC output for +/-500VDC rails

Sumr part #: RC0100 050 3

100VA / 117V primary  230 V primary

2 x 450V (@125mA output ea) secondaries

1 x 30VCT (15V-0-15V) 10VA secondary

Core band

Static shields

 

Would the correct specs be 4 x 72 VCT (36V-0-36V) @ 2A each secondaries? 

 

Yeah, I'll be using 2 PSU-boards.

 

/V

Edited March 11, 2013 by Valentin Hogea

[jwzhan]

oh sorry man... I was talking the Dynahi PSU.. because this is the dynahi build thread, so I assumed... forgot that you are building the KGSSHV (or are you building both at the same time?)

One PSU board for both channel in KGSSHV's case.

 

Those specs for the KGSSHV transformer are fine. You can up the output current for the HV winding to... say 200mA (instead of 125mA,) and raise the output current of the amp. This will add some heat, but should make the amp sound even better. Extra heat should be no problem with off-board version, because those heatsinks are way overkill. (mine runs at sub 40C level with default current output) But don't do it if you are building the on-board version, because that runs plenty hot with default output.

 

John raised the output current and he is very satisfied. Details are in the KGSSHV thread.

Edited March 11, 2013 by jwzhan

[Valentin Hogea]

Hi Mate!

 

I'm building both... Thx for the update on the KGSSHV trafo. Haven't ordered it yet, and yeah... I'm doing the off-board version. 

 

 

So  what specs are correct for the DynaPSU? I used the KGSSHV detailed specs as a "base" that you could explain from...

[StrawHat]

I'm using a single 160VA 2x30V transformer to power both boards and have now two full working psu's.

I didn't populate the lower voltage sections for now as I won't be using them and still haven't applied the current limiting transistors fix

 

Anyway both psu's are rock solid at +/- 30V for now but I'm having lots of problems biasing and correcting the DC offset on the amp boards. I think that's what I get for not doing any matching

[jwzhan]

Hi Mate!

 

I'm building both... Thx for the update on the KGSSHV trafo. Haven't ordered it yet, and yeah... I'm doing the off-board version. 

 

 

So  what specs are correct for the DynaPSU? I used the KGSSHV detailed specs as a "base" that you could explain from...

 

transfo for Dynahi is as I described.

160VA for two amp boards are fine... as long as you only drive headphones.

I would still use the one I said earlier. 72VCT@2A (148VA) per channel...

Make it a two windings in one or just get a 72VCT@4A and it shouldn't cost much more than a [email protected] transfo if you are getting a custom one.

 

overkill is always good.

 

I think Kevin mentioned some where in the dynalo project page that the opamp is only able to offset something like 10%.. so some careful parts matching are definatedly needed.

 

EDIT: to be more precise.. a transformer with 2 x 72VCT @ 2A (or 1 x 72VCT @ 4A) for the entire balanced dynahi amp is recommended.

Edited March 11, 2013 by jwzhan

[Valentin Hogea]

transfo for Dynahi is as I described.

160VA for two amp boards are fine... as long as you only drive headphones.

I would still use the one I said earlier. 72VCT@2A (148VA) per channel...

Make it a two windings in one or just get a 72VCT@4A and it shouldn't cost much more than a [email protected] transfo if you are getting a custom one.

 

overkill is always good.

 

I think Kevin mentioned some where in the dynalo project page that the opamp is only able to offset something like 10%.. so some careful parts matching are definatedly needed.

 

EDIT: to be more precise.. a transformer with 2 x 72VCT @ 2A (or 1 x 72VCT @ 4A) for the entire balanced dynahi amp is recommended.

 

So 2 x 72 VCT @ 2A with extra windings for driving both PSU boards?

[jwzhan]

no extra windings. just two secondary windings with 72VCT @ 2A each.

 

Even if you want to have the lower voltage rail to work for the digital attenuator, you should have it in another transfo, because you need to have the controller module on 24/7 for it to remember your settings, so the transfo for those rails should be separated from the transfo that provides power to the amp. You don't want to turn-off the controller with the amp and come back to find that all your settings are restored to default because the cap that's sipposed to be providing power to memorize the settings ran out of juice.

If you are not using a digital attenuator... just leave the lower voltage section unpopulated and save some money.

[StrawHat]

So I wanted to ask some advice to help me set my dynahi

 

I can't set the offset to 0 it's just to unstable. On startup without any adjustments to the trimpots the offset is on the volts scale (1~5). With the servo connected or disconnected it's the same I can adjust it with lots of patience to some milivolts (10~30) but then out of nowhere it just jumps up to 5v again.

I replaced the 500R resistors with 1k to be able to adjust the bias current. The only different parts on the amp are the 2pf caps because I could find only 2.2pf that could fit the holes.

BTW to anyone using the spreadsheet BOM those 2pf caps have the leads too big for the board they need to be changed. Same problem with the opamp sockets.

The only matched transistors I have are the input jfets and the ccs 2sa1145/2sc2705.

 

Thank you for any help you can provide.

[kevin gilmore]

make sure you have a pot connected, or the inputs shorted to ground.

[StrawHat]

I tried with the inputs shorted to ground with 1k resistors after reading an old post from Ti Kan on the original HF dynahi thread but it didn't help

[Pars]

How is your heatsinking? The output devices need to be thermally stable, although even with that I don't know that you would see 5V output DC offset. I'm not sure if there is an operating points diagram floating around, but it sounds like you may have a wrong part somewhere. Also, the LEDs should be matched pretty closely for V drop.

[spritzer]

A marginal part would also account for this as the amp jumps from normal to fubar.  Not probable if both channels act this way though. 

[Pars]

Agreed. The bouncing around part and the amount that it is moving seems to indicate a problem. For something like an I/V stage with no feedback, I see this. For a dynalo/dynahi, they normally seem pretty stable for offset if things are correct in the build, pretty stable meaning it should be easy to get it to +/- 2mV drift without the servo in place..

[StrawHat]

I checked and rechecked every part I'm pretty sure I populated everything correctly.

I'm starting to suspect I may have some fake transistors. I bought all the transistors form reichelt.de except the matched jfets which I bought from here:

 

http://stores.ebay.it/Himmels-Rohren-Shop?_trksid=p4340.l2563

 

They all seem to be real though.

[spritzer]

Fakes would explain it.   At this point I just strip all the active bits and start anew....

[jwzhan]

I'm just curious. Do you have any photo of the build?

From what you said earlier, does the opamp feedback circuit do nothing at all?

Might be fake parts if it's happening to both channels. Did you buy spares for all parts? I would test them.

 

For the BOM, I've fixed most the the problem, including opamp sockets and the 2pF cap.

 

EDIT: Birgir beat me to it.. yeah.. I was also thinking of the same thing.. I wouldn't even bother with stripping the board though, because the passive components are cheap anyway.. Start anew with new boards is the best way, because it definately is easier... Hopefully you bought enough spares.

Edited March 15, 2013 by jwzhan

[Pars]

The DC servo really isn't a feedback circuit. However, thinking about it, you do have the negative feedback in place, correct? Don't have the resistor numbers offhand, and I think the 2pf cap is compensation for the feedback circuit.

 

You might just pull an active of each type, or if you have spares, test them.

 

EDIT: R52/53 and 56/57 are the feedback circuit, with C1 and C3 as compensation caps (schematic shows 1pf).

Edited March 15, 2013 by Pars

[spritzer]

The servo is limited in its range so if the trimpots can't even get close to 0V then then servo will be useless. 

 

Matching is important but I did build a Dynalo with no matching at all (except for grade) as a test and I had no problem getting it down to 0V.  I didn't even install the trimpots, just used drop in resistors like I did on my last build. 

[RudeWolf]

Even if you want to have the lower voltage rail to work for the digital attenuator, you should have it in another transfo, because you need to have the controller module on 24/7 for it to remember your settings

I'm not sure about which type of volume controller you are talking about, but in my Jtree attenuator all the remembering is done by a potentiometer. The controller just reads the impedance and sets the relay configuration accordingly. Even if I power down the pot still stays in the set position and when powering on the controller sets the correct volume step. However I still used a separate power transformer for the digital circuit. Seemed like the kosher thing to do.

[jwzhan]

I'm not sure about which type of volume controller you are talking about, but in my Jtree attenuator all the remembering is done by a potentiometer. The controller just reads the impedance and sets the relay configuration accordingly. Even if I power down the pot still stays in the set position and when powering on the controller sets the correct volume step. However I still used a separate power transformer for the digital circuit. Seemed like the kosher thing to do.

 

That's would be KG attenuator with LCDuino controller. I emailed Ti Kan on the matter for confirmation and that was his response. There is a super-cap on board that keeps things remebered (not only volume position, but also other settings such as time and input selection if you also have that board,) but only for a certain amount of time. Similar to how the motherboard remembers the BIOS setting. If the cap runs out of juice, the settings will be cleared out.

 

Anyway, by leaving the attenuator powered on 24/7 and keep the wires short, it can also double as a separate volume control with the main amp and its PSU off... When I finish mine, I will probably put it in between KGSSHV and 1600HD and just turn the Alpha pot to max. The Alpha pot is not really giving me much control over the KGSSHV's volume.... I've never turned the pot pass 20 degree... it just gets way too loud

 

hmm.. I thought the DC offset servo is a part of the feedback... I guess I was wrong all along. Thanks for the info.

[Pars]

Re: servo

There was an amusing thread on HF where Rudi (rudistor) got into it with Kevin claiming that the servo was feedback and bad... the servo operates under 1Hz, so I guess it is somewhat

[spritzer]

The gain of the HV is a bit on the high side but it can be lowered. 

 

Ahh Rudi, as braindead as they come.  Remember the "independently tested" specs he posted?  Turns out he was in good company with Cavalli on that one... 

[eggil]

It is high, and I wouldn't mind trying a lower gain.

Has anybody done this with their HV?

Edited March 16, 2013 by eggil

[jwzhan]

You will be amazed by the number of people who think Rudi is a genuis in Asian forums. And frankly if anyone says otherwise, he will simply be flamed or silenced. The funny thing is.... almost no one has listened to the shoddily made amp or see the measurements themselves.. They reasoned that Rudi amps are expensive and people who buy them are not braindead.. (or are they?) enough said... I'm glad I don't see Rudi in any of the English forums that I visit now.. I guess I owe my thanks to Birgir and Kevin..

 

I think I will stick with the default gain with HV for now, but I'm still interested in knowing how to lower it.