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and now for something completely different part 3


kevin gilmore
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7 minutes ago, MASantos said:

I got my case today. The power switch is rubbish and I'll try to get an illuminated pushbutton with European approval that fits the hole. 

Hunting for power switches is a nightmare, unless you're willing to go with a bulgin vandal-proof type of thing. I'm mounting my power switch on the bottom plate near the faceplate, so you can reach under the amp and easily push the toggler switch.

10 minutes ago, MASantos said:

Does anyone know of nice looking 1/4 Jacks other than neutrik?

This is also pretty okay - https://eu.mouser.com/ProductDetail/Amphenol-Audio/ACJM-MHOP?qs=t8VhaDIDl4uYvnn1Z02tXA%3D%3D

But generally the choice isn't too big.

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17 hours ago, thuytn said:

How did you adjust the input trimpot? Also, what’s the part number of the 47uf ceramic cap on the CFA3 are you using?

I adjusted the trimpot to nullify DC on input board outputs, before the servo opamps got installed.

For the ceramic cap I used this - https://eu.mouser.com/ProductDetail/81-RDEC71E476MWK1H3B

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On 6/1/2020 at 10:26 PM, RudeWolf said:

Hunting for power switches is a nightmare, unless you're willing to go with a bulgin vandal-proof type of thing. I'm mounting my power switch on the bottom plate near the faceplate, so you can reach under the amp and easily push the toggler switch.

This is also pretty okay - https://eu.mouser.com/ProductDetail/Amphenol-Audio/ACJM-MHOP?qs=t8VhaDIDl4uYvnn1Z02tXA%3D%3D

But generally the choice isn't too big.

I had the under switch in my power amp, but this one wont have feet high enough to reach under.

I'm ordering a latching anti vandal switch and will be using a 4 pin xlr even though the amp is single ended. 

I already have my hd650 with a balanced connector so I wont have a need for adapter cables.

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So I wired the amplifier board to the GRLV.

One channel fires up ok. Both leds giving light, about 150mV across the output resistors 90mA offset without servo and input open.

On the other channel I have one of the LED not lighting. No reading on output offset.

all the LEDs test ok with a multimeter I diode check and emit a small light. 

On the mjf15030, the output resistor reads 30mv across. On the 15031 nothing across the output resistor. 

I don't see any solder bridges. Should I start replacing stuff?

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Edited by MASantos
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So I cleaned the boards once again and checked every transistor for orientation and location. Both boards match. 4 LEDs appear to be ok, similar voltage drop.

I powered both boards and took some random VDC readings to ground. This is where things show that something is wrong. Most of the small critters have different readings. They all came from the same 100 unit strip bought from mouser. I guess I'll go ahead and start replacing them all.

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Did the diode across the board and found some discrepancies in the input pair. Replaced both critters and voila. Board is biasing normally, both LEDs are shining. 60mV of DC offset on both channels with inputs shorted. Will finish wiring inputs output while waiting for servo opamps. 

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I biased mine (balanced) at around 100mA, and didn't notice any sonic improvement from 80mA up actually. 

But please take my view with a pinch of salt  as I am using a small on-board heatsink and is hitting around 55 deg C at 100mA. Output devices used were MJE15034/5

 

That being said, do you guys notice improvement jump at around 150mA?

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Just a few words on the sound of the balanced CFA (with single sigma22, mje15034/5 output transistors, default gain).

Fed by Susy dynalo (thru-hole version) with GVLR PSU as DAC buffer.

I think "natural" would be apt to describe the sound of CFA. The timbre is simply natural sounding without exaggeration of any sort. The sound stage is expansive yet the imaging isn't smeared. Using dynalo>>CFA as compared to using dynalo alone makes the soundstage more expanded (less 'in your head' feeling when it comes to vocals).

The background is dark (unlike dynahi), and the overall signature is neutral and less warm compared to dynahi. CFA doesn't play nearly as loud as the dynahi at default gain, but it plays with class and authority. It is simply the best sounding amp I've heard, natural tube-like sound without any obvious colouration. Thank you so much Dr Gilmore!

Next step - split chassis and dual GRLV PSU.

 

PS: attached is a preview of PCB (sans ground pour) I made according to the gerbers and schematics. 8cm*10cm. compatible with the dynahi heatsinks. Probably very amateur but it works. I don't mind sharing if anyone wants.

 

 

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So when I got my case I got pretty upset with the looks of the power switch:

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So I search around the house for what could be used to improve this and found and unused aluminum kitchen drawer handle. Nice looks, hollow and easy to cut. A couple of minutes later:

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It's alive! Finished wiring today, only thing missing is drilling the front panel for the 4 pin XLR, waiting on a drill bit. 

Probably the best amplifier I had for the HD650. Absolutely dead silent, great sound! I'll need to make a better comparison with the dynalo mini, but at first glance it sure seems like a winner.

Thanks for all the help I got during the build!

 

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24 minutes ago, Pars said:

The case looks decent... something from ebay/aliexpress?

It looks like the switch was drilled for a Bulgin type vandal switch...

The case is alright. Not top notch but also not a bad price considering it has most of the drilling done. 

The whole for the switch is 13mm wide with a little bevel on the front. The cap on the pushbutton didn't fit right so I made that cosmetic upgrade. 

I am considering painting the front panel mate black. 

 

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