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Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 28th September 2017, 01:37 PM   #2141
Algar_emi is offline Algar_emi  Canada
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Default DCG3 Q5-J3 silkscreen identification on PCB

I cannot find J3 on the DCG3 pcb I received. There is a Q5, not listed into the BOM that seems to be the correct part. Is there an error on the pcb silkscreen?

SB
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Old 28th September 2017, 01:42 PM   #2142
Tea-Bag is offline Tea-Bag  United States
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Quote:
Originally Posted by Algar_emi View Post
I cannot find J3 on the DCG3 pcb I received. There is a Q5, not listed into the BOM that seems to be the correct part. Is there an error on the pcb silkscreen?

SB
Yes, there is an error - Q5=J3
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Old 28th September 2017, 02:45 PM   #2143
Algar_emi is offline Algar_emi  Canada
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Tx. SB
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Old 2nd October 2017, 07:39 PM   #2144
dbis is offline dbis  United States
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Default AC primary fuse size and inrush limiting thermistors

Hey everybody,

I was able to get back to my DCG-3 build this weekend but a couple questions came up:

1. Based of diyaudio.com searches I've seen that the formula for AC inlet primary fuse size is power/AC voltage x 3. So for me, with a dual mono DCSTB with 2 x 50VA Antek AS-0518 transformers, I get 50VA/120VAC = 417mA x 3 = 1.25A. That's per transformer so I'll double that to 2.5A for the fuse. Did I get that right? Seems kind of high to me. I tried to get this info from the Antek spec sheet but that didn't really help me or I didn't read it right.

2. On the power amps I've built so far, F5 and VFET, I've used the standard Pass/diyaudio store PSU and followed the schematics with the two inrush CL60 thermistors. I thought I might not need to do that on this preamp but I'm seeing some of your builds are using them. Is that something I need to do or just a nice-to-have? I do have the parts on hand so I can do it if need be.

Thanks,
Dan
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Old 2nd October 2017, 11:28 PM   #2145
Salas is offline Salas  Greece
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1. That's right because the inrush current during the first half cycle can be many times the rated primary current (especially in toroids)

2. Nice to have, but with such protection you should also cut the AC inlet fuse rating at about half the above
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Old 3rd October 2017, 12:17 AM   #2146
dbis is offline dbis  United States
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Thank you Salas.
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Old 5th October 2017, 02:39 PM   #2147
Algar_emi is offline Algar_emi  Canada
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Default Selection of the best uPA68H IDSS

Hi. I bought 5 uPA68H and I tested each of them for correction operation (not fake) and IDSS. I got these results, which pair should I select for the amp?

#1: 2.78 / 2.75 ma
#2: 4.46 / 4.50 ma
#3: 12.8 / 12.8 ma
#4: 8.0 / 8.0 ma
#5: 5.9 / 5.9ma

Intuitively, I would choose #2 & #5, both over 3ma, and the closest.

Thanks
SB
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Old 5th October 2017, 02:59 PM   #2148
Salas is offline Salas  Greece
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In circuit all of those would be biased at about 2-2.5mA per section by the tail CCS (BF256 degenerated). Dynamically they will swing very little current about that point (0.025mA pk-pk each for 12V pk-pk output signal). If you look at various types audio JFET curves they tend to converge for transcoductance around low current bias points no matter how higher they can go for IDSS as individual samples. But use #2 & #5 as you say so their Vgs (off) is closer too. Although the current mirror presents very high impedance loading so the pinch off will not differentiate things appreciably. In general I have designed around the uPA in a way to avoid samples waste. Their section to section curve tracing match is great by NEC in all IDSS I have tested by the way.
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Old 5th October 2017, 03:38 PM   #2149
Algar_emi is offline Algar_emi  Canada
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To resume 2 & 5 are ok Testing the circuit at this very moment, everything seems fine.
Here the few measurements I did, Supplies +/-17V from bench power supply at the moment, and no load:

Vsupplies +-17V, Isupply +260ma, -250ma
Input pair bias = 5.3ma (0.8V accross R3, both Channels)
Bias: 115ma, on both channels
Vin max: 3.7Vac, for Vout max: 11.5Vac (1kHz)
Gain for Vin=1V, Vout=3.12V at 1Khz = 10.1dB, same on both channel
Vdc out < 1mv after a few sec. (DC offset adjusted without U1 plugin, then inserted U1)
BW (Ref 1V, 1Khz): 3.2Mhz!, but without the input volume pot, and no load...
THD (Ref 1V, 1Khz): 0.006%, or very low...

Waveform is very nice, no instability, time for a listening test...

Last edited by Algar_emi; 5th October 2017 at 03:59 PM.
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Old 5th October 2017, 04:00 PM   #2150
Salas is offline Salas  Greece
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All the above looks well. There is considerable load already, the feedback network driven is much lower impedance than hooking power amps. Is your output stage bias testing very stable too as usual in this circuit? That contributes to low memory distortion. Will sound good enough on a lab bench psu but it will sound better on a dedicated low noise low impedance psu and short wiring.
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