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The Multi Amp aka Dynalo Mk2


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You're right, I had wondered if those LED's were wrong.  I've swapped them for what you posted.  For the 5pF I believe it's 5mm but will measure again tonight.

Back when I built the original Dynahi I had to swap out LED's to get the lowest offset IIRC.  Is that the case here?

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Here's an update on the mini. I've got the external regs running now.  This thing does throw off quite a bit of heat.  I've some sinks on the output transformers now, but I'd like to make them a

Almost there.  I've got one channel dialed in and working on the second.   

Well that was very quick shipping from FPE , Initially I was going for an all out black box, but FPE don’t do anodising after milling leaving the panels with silver edges, which got me thinking s

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The 5pf caps I had in my BOM were Mica (598-CD17CD050DO3). Kevin's BOM (old one) used COG ceramics (81-RDE5C1H5R0C0K1H3B).

As for the 20R resistors, try them as long as they are appropriately sized. I'm not too much of a fan of the PRPs anymore (coating comes off easily). Never used Takmans. I've been using the Vishay/BC MRS25s for some stuff and have liked them.

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11 minutes ago, sbelyo said:

should I socket the LED's?  I remember doing this on my original Dynahi to get the offset as low as possible.

Not needed here because the offset is trimmable here - it wasn't on the original Dynalo.

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  • 4 weeks later...

My dynalo mini started acting up. Balanced output works fine, but SE output had low volume and distortion.

I did another thorough clean up with alcool, double checked everywhere for solder bridges and it started working again, probably some bridge of flux I didn't spot before.

It now has almost even level between channels, but just a bit lower level on the left channel. It could almost be due to a potentiometer inbalance but it is perfect in balanced output.

 Dc is 0.1-0.2 on the right channel and 2.4-2.6 on the left. When starting up the bias goes down quickly on the right channel but take a bit longer on the left one. this is with servos in the circuit path. I took VDC measurements around the board on most transistors and servo opamps and I couldn't spot any differences between the 4 channels. 

Any ideas of what might be causing this? Thank in advance for your help!

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1 hour ago, MASantos said:

Dc is 0.1-0.2 on the right channel and 2.4-2.6 on the left. When starting up the bias goes down quickly on the right channel but take a bit longer on the left one. this is with servos in the circuit path. I took VDC measurements around the board on most transistors and servo opamps and I couldn't spot any differences between the 4 channels.

I assume that the values shown are mV? Also, when you say bias, I assume you mean offset?

The first place I would start would be to measure the bias voltage across all of the 20R resistors, noting any variances. I would then do a comparison of the biasing circuits (consult the schematic) and go from there. It can be difficult to spot solder bridges on these, so checking resistance across all resistors (power off) might show you something. I know I had a resistor shorted that looked fine.

Edited by Pars
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12 hours ago, Pars said:

I assume that the values shown are mV? Also, when you say bias, I assume you mean offset?

The first place I would start would be to measure the bias voltage across all of the 20R resistors, noting any variances. I would then do a comparison of the biasing circuits (consult the schematic) and go from there. It can be difficult to spot solder bridges on these, so checking resistance across all resistors (power off) might show you something. I know I had a resistor shorted that looked fine.

Those are dc offset mV indeed. long day yesterday...

I'll take readings on the resistors and check the biasing circuits to start!

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So I found a dead resistor in the R+ channel and replaced it. The L+ channel is the one acting up.

I removed the servo jumpers and dialed the offset to less than 5mV on each channel. All fine. When I reinstalled the jumpers the L+ dc offset jumped to around 350mV while all the other channels remain below 2-3mV (but still wondering between 0 and 2-3mV, not ultra stable). 

The servo opamp for the L+ channel is getting power as it should. Might the IC be damaged and causing this?

On further inspection, the 100k resistor next to the opamp measures 28K when soldered (I removed it and it measures 100k as it should) 

The opamp is measuring 1.1ohm between pins 2 and 3 (input + and input-) this is different from all the other opamps so I'm ordering a new one. 

Edited by MASantos
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I replaced the opamp and now it reads as the other 3. The offset still drifts around below 10mV. All other amplifiers I built (cfa, ckk3) with dc servos were very steady once the servos were installed. 

Is this drifting normal in the dynalo mini? I can't seem to find any solder bridges, it is as if the servos are not doing their job at all. 

Sound is now centered and similar in both SE and BAL ouputs, it is just the drifting that is puzzling me. 

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John, those should be fine. Comparing the datasheets, it appears that thermal performance has decreased ( or been reevaluated), but electrically the same.

Regarding the Dynalo, yes, they do seem to drift more (and act like the servo isn't working). IIRC, even the full size ones do this.

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I can confirm that the full-size SuSy Dynalo offset does drift even with servo engaged as Chris stated.

My own observation:

1. The drift is mostly ambient temperature related. A cased amp drift less than a free-standing (aka air-chassis) amp does

2. The + out and - out drift the same amount so, while the offset relative to ground drifts, the voltage difference between + out and - out are fairly stable 

Edited by mwl168
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