KG Balanced Dynahi build discussion thread

[Pars]

Had to refresh my memory on the original dynalo/dynahi. Yes, you should replace those resistors with the 620/680R to give yourself adjustment range. What do you have in for the bias resistors (R23/R34)? The schematic I have shows 100R, but I think most use 500R?

[jose]

Yes, you can change r500 (next to Trimpot) for other from 620 Ohm to 1K.

[penmarker]

6 hours ago, Pars said:

Had to refresh my memory on the original dynalo/dynahi. Yes, you should replace those resistors with the 620/680R to give yourself adjustment range. What do you have in for the bias resistors (R23/R34)? The schematic I have shows 100R, but I think most use 500R?

 

5 hours ago, jose said:

Yes, you can change r500 (next to Trimpot) for other from 620 Ohm to 1K.

Thank you both so much. @Pars are the R23/34 the resistors next to the trimpots? If so I have them at 500 Ohm according to the board. I have the board with this layout, which is the dynahibal9.zip in the staxpcbs Google Drive folder.

Regarding my other points in my previous posts, why did the output offset suddenly shot up to 5V? That amount was insane.

I've carefully set the trimmers flanking the THAT340 in their mid point with my DMM before soldering them in and haven't touched them yet. The moment the offset shot up I switched it off and scrutinised all solder and tried smelling all devices but nothing looked or smelt burnt.

I turned the lower left/right trimmers all the way down again and turned it back on, and the offset was only 1mV. Then I tried biasing them back up to 270mV and the bias stayed low. I switched it off, and then after a few minutes switched it on again and the offset shot back up to around 3-4V and drifted down to several hundred mV. I repeated this with min 140mV bias, switching off/on, working all the way up to 200mV where the offset stayed low without displaying the erratic behaviour.

Both L/R trimmers are turned up in half turn increments at the same time while watching both the bias voltage and offset voltage.

Inserting OPA445 makes the offset drift more than without. Probably because my devices are matched well?

Edited March 19, 2018 by penmarker

[Pars]

No, not the ones by the trimpots (I don't think). This is the schematic I have; not sure where R23/R34 are on the board layout.

I would definitely change out the 500R by the trimpots (R1 and R4 on the schematic) with the 620R or 680R. I see that there are 1K resistors in series with the trimpots; even so, you cannot get the effective resistance of 11K || 500R above ~478 ohms. With 620R, you can get it to about 586R.

I wouldn't think that putting the OPA445 in should have the negative effects that you are seeing.

 

dynahibalbjt.pdf

[penmarker]

I'm positive R23/R34 are 50 Ohm resistors on my board.

50 Ohm is only 10% of what most people use. Should I increase this to 500 Ohm like other people along with replacing R1/R4 from 500 Ohm to 680 Ohm?

[Pars]

I'll have to defer to those that have built this version of a dynahi as I have not.

[penmarker]

Thanks a lot Pars, at least I have a direction about where to poke and prod. I've contacted someone who had built using this board too, hope all is well.

[Helium]

6 hours ago, penmarker said:

I turned the lower left/right trimmers all the way down again and turned it back on, and the offset was only 1mV. Then I tried biasing them back up to 270mV and the bias stayed low. I switched it off, and then after a few minutes switched it on again and the offset shot back up to around 3-4V and drifted down to several hundred mV. I repeated this with min 140mV bias, switching off/on, working all the way up to 200mV where the offset stayed low without displaying the erratic behaviour.

Why are you biasing to 270mV only? With off-board heatsink, dynahi must be biased to 750mV (and even more if you like and as long as temperature is OK). With onboard heatsink, half of that (375mV) or more as long as temperature is OK.

[rumina]

i used some sip sockets for the 500 ohm resistors near the 10k pot, makes the change easy. the value of the 500 ohm resistor depends on the vf voltage of the leds you are using, i ended up with some 374 ohm resistors for 620 mv bias (due the relative small case).

Edited March 19, 2018 by rumina

[penmarker]

2 minutes ago, rumina said:

i used some sip sockets for the 500 ohm resistors near the 10k pot, makes the change easy. the value of the 500 ohm resistor depends on the vf voltage of the leds you are using, i ended up with some 374 ohm resistors vor 620 mv bias (due the relative small case).

I guess it's my fault because I ordered the LEDs off element14 along with the other parts and they were rated at 1.7V Vf. I didn't check and populated the board with the LEDs.

So since you lowered the resistor to 374 ohm, does that mean you can't get a low bias voltage with the stock 500 ohm?

[rumina]

i had the same as you, couldn't get over 300mv, then i tried 620ohm - even lower bias. so i tried the other direction ;-). sip socket makes that a lot easier, i solderd them in after the desolder of the 500 ohm resistors.

[penmarker]

That sounds peculiar but I can't argue with results. What about R23/R34? Did you need to increase that from 50Ω?

[jose]

Before touching anything, I would first change the resistance of 500 Ohm.

About offset, In my case the Offset was influenced by the BIAS too high and by the THAT340.

 

Edited March 19, 2018 by jose

[Pars]

Because of the 1k in series with the pot, the lowest you can go with the pot at minimum is ~333 ohms (1k || 500R). Normally, the 500R is replaced with a 620 or 680R because you can’t go high enough in resistance. It appears that on this amp, the problem is going low enough? If that is the case, replacing the 500R won’t do anything to solve this.
I was looking at R23/34 because in the Ss Dynalo, there aren’t trimpot such as you have here. These resistor positions are what is used to change bias, unless I am not analyzing the schematics correctly.


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[penmarker]

Alright for safe measure I've ordered a strip of sip sockets as well as a bunch off different value resistors. I'll try and see if I need to increase or decrease the 500 Ohm resistors and will report back maybe in a few days or so.

[Pars]

I'd adjust for max or close to the bias you are looking for (750mV??) and then measure ohms across the trimpot(s). Even though they are parallel to the 500R, you should be able to tell approximately what end of their travel they are at, and go from there as to what needs to be done.

[penmarker]

How much current will the amp pull in total?

I want to get a proper rated fuse for it and check whether my Schurter IEC inlet is the proper one. I bought this 2A version way back then without checking first.
http://my.element14.com/schurter/3-101-141/iec-inlet-c14-c18-with-filter/dp/2671668

[luvdunhill]

A mains fuse should be rated for the transformer, regardless of how much current the amplifier draws.

[penmarker]

I'm using a 160VA 2x30V, so that means 160÷2÷30=2.67A?

[dsavitsk]

Rate the fuse for the primaries.

[gepardcv]

@penmarker: This is the standard HC advice on the subject: 

 

[penmarker]

Thanks that's very helpful.

[penmarker]

Oops, I have to echo some of the alternative findings here. The bias goes down instead when I replaced the 500Ω to 680Ω. Got them on sip sockets to change quickly

I'm gonna go out and get some lower resistors now.

[Helium]

Is that clear type LED 1.7Vf? It must be 1.7Vf (usually red color).

Fill with solder all vias. Helps to reduce overall impedance of PCB.

Edited March 24, 2018 by Helium

[penmarker]

Correct, they're red and rated at 1.7V.

Replaced them with 360Ω and now can go to 750mV