Salas hotrodded blue DCB1 build

It surprises me that audio preamplifiers do not adjust for these differences for each input... such as like what one sees on a pro audio mixing console. I guess faders are more like attenuators, but I think you will understand what I'm saying.

My initial question about a gain stage was a recommendation for a simple solution to add gain for solely the phono stage in something like the dcb1.

Disregard the comment about the cd input- seems that's totally off base... I have some more reading to do.
 
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Many commercial inputs have selectable input sensitivities for each input channel.
Some are quite ordinary domestic HiFi separates.
But it costs money to incorporate these and you will find that the cheapest either don't incorporate or do it so cheaply to remain price competitive that performance is compromised.
Some digitally controlled amplifiers use microprocessors to adjust the input sensitivities.
 
Yes I understand now, gain precedes attenuation. A potentiometer does not reduce gain but limits it, yes?..............
No,
gain and attenuation are different.
You generally have a fixed gain. It's the attenuation that you choose to change/adjust.

If you have a 2Vac source like a CDP and your amplifier needs 2Vac to just reach clipping then when the full output signal (no attenuation) is sent from the CDP the amplifier only just reaches clipping on the highest transients. The average level from the CDP is much lower.
When you adjust to add in some attenuation, you find that the highest transient is well short of clipping the amplifier. This is the situation for much of our listening.
I find that I listen with the vol pot set to between -40dB and -20dB
The highest transients are then 10dB to 20dB below clipping.
If I turn it up "loud" then the amp will be closer to clipping. I hope my amplifier never clips.

Now take an amplifier that only needs 1Vac to reach clipping and use sources that have 2Vac maximum output.
Your attenuator should never be set above -6dB. That might be about 60degrees to 90degrees down from maximum rotation of a 270degree adjuster.
This is a system with excess gain. Not much good to anybody. Except if you have a low output Source. Then the extra gain would allow you to reach clipping when you want to listen loud.

If you have a range of sources with a widely different maximum output, then it would make some "system" sense to apply some extra gain ONLY to the sources that are at the lowest end of output capability.
This is where a +6dB or +12dB gain stage can be incorporated. it is switched in when you choose a low source voltage AND you want to listen loud.
 
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DCB1 Voltages check

Hi All,
First post here!
I must firstly say a Big Thank you to Teabag, Salas, Andrew and all the other excellent people that make these forums so interesting and informative, and without them I would not have gotten this far!

I finished a DCB1 a week ago but my voltages are -9.75 and 9.76 but I see they should be over 10 and most peoples are.
The offset is 0.4mv and 1.5mv
The amp sounds good but is lacking gain, as in I have to turn it almost full to have an normal listening volume! I have the 10ohm resistors in but not much heat!
Should I just leave it and use or make adjustments?

The plan is for it to be part of an integrated amp, the power side are two Hypex UcD400hxr Class D modules. See pic. Just wondering if anyone here has done something similar and are they happy with it!
Aubrey
 

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Welcome to posting and congrats for your builds. Your rails and offset measurements are well within range of DCB1 spec, there's nothing to rework for any real practical improvement.

To have enough volume its about the total system gain thing. You should verify your amplifier's board mode for its input opamp engagement jumpers and the gain needed. The correct input signal wiring from a single ended source also. From the manufacturer's literature or by directly consulting with them. You should be able to achieve adequate gain. Mention your speakers sensitivity.

The DCB1 adds no gain in the chain.
 
Voltage Checks

Thank you Salas for such a fast reply and the confirmation of the readings!
I'm using a pair of Altec 604-8G in 620 cabs, these are very close to 100db as they come from the Tube era!
I'm beginning to think the Altecs are not really a suitable match for the Class D amps, but they work well with a little NAD3020 so a little bit of experimenting is on the cards.
Thank you again,
Aubrey

An externally hosted image should be here but it was not working when we last tested it.
 
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For your speakers and a digital source when followed by a unity voltage gain DCB1 seek around 15dB of SE mode power amplifier gain. If having a phono stage also, maybe 20dB. Contact the Hypex company for info to can examine bad chance this power module is set at minimum 13.5dB balanced mode and halving it when driven from an RCA output. Maybe it has various gain options on-board. If its fixed 26dB BAL then it should be able for at least a usable 13dB SE but make sure the now wiring from DCB1 to Hypex isn't wrong in general or antiphase between channels so the speakers don't cancel some acoustic power in room.
 
My FSP Phono adheres to the circa -10dBV (300-350mV) old commercial output standard. As most phono stages do. Your now preamp that you want to upgrade on has 120mV line level sensitivity for 1V output. That's 8.33 times gain or 18.4dB. I recommended you a DCG3 for line duty instead of a DCB1 after the FSP and to back down your DAC a little. Has 3x or ~10dB gain. 3x330mV = 1V. I don't see where I recommended you extra gain from what you now got. With your now preamp the magic gain sum number is 18.4+25+87=130.4 (line+poweramp+speakers). Your then magic number will be 10+25+87=122. The "around 120" ghost is showing up again :)



Would it be sensible to use a secondary gain stage within a phono preamp if this is the only source which requires it? Then it could feed into a passive or buffered preamp?


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Salas, others: could you comment on this snippet and how it might relate to our purposes with your transistor designs such as the preamplifiers and MM/MC stage? I found when researching the scp-2 vendetta research phono preamp.

"Footnote 3: John Curl hand-selects FETs for the SCP-2 with a transistor tester which not only measures noise but classifies that noise according to frequency. Thus he can select both on the basis of low noise and of noise with a particular spectral density.—John Atkinson"
Read more at http://www.stereophile.com/content/...phono-preamplifier-page-2#Hmhs6AKYpIZUm3cs.99

Transistor technology is totally new to me. All of my other builds and mods have been in the tube domain, which seems to me considerably more direct and understandable than solid state technology.

I'm wondering how to extract the best performance and if it would be wise to try and match transistors in this manner?


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Is it possible to bypass the shunt?

Hi all,
after more then 1 year of (happy) listening, my board has begin to make strange things:

at beginning (2 weeks ago) the 5LED group on V- rail was ON at power on and after a while they went off (actually all V- rail went off) and sometimes went on again without any appearent reason...

nowdays the 5LED NEVER light on and so all the V- rail (V- = -22v)
I'm sure the 5LED are ok becouse they still light up (for fews seconds) when I power off the board.

the V+ rail is ok (V+ = +10v) it is working at moment BUT yesterday for the first time the problem started ALSO on this side of the board :(

can someone please tell me what to check to know which component has to be replaced?
tnks'
Saverio

Well after some checks it seems that I completely destroyed the board while soldering ... So I think the only solution is to buy everything new and rebuild from scratch.. :(
The question now is: is it possible to use what remains of the board (the PSU (15-0-15 trafo), the regulated relay mute and B1 buffer seems ok) bypassing the shunts regulators?
Thanks,
Saverio
 
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Yes, its rather well separated by different functioning areas on PCB, there are even DC +/0/- pads that can accept a connector at the end of the regulator section. Use the continuity buzzer of a DMM and some hobby knife to completely separate what you don't need
 
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Salas, others: could you comment on this snippet and how it might relate to our purposes with your transistor designs such as the preamplifiers and MM/MC stage? I found when researching the scp-2 vendetta research phono preamp.

"Footnote 3: John Curl hand-selects FETs for the SCP-2 with a transistor tester which not only measures noise but classifies that noise according to frequency. Thus he can select both on the basis of low noise and of noise with a particular spectral density.—John Atkinson"
Read more at Vendetta Research SCP-2 phono preamplifier Page 2 | Stereophile.com

Transistor technology is totally new to me. All of my other builds and mods have been in the tube domain, which seems to me considerably more direct and understandable than solid state technology.

I'm wondering how to extract the best performance and if it would be wise to try and match transistors in this manner?


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http://www.diyaudio.com/forums/pass-labs/175040-more-fet-noise-measurements-euvl.html
 
Completed building the Hypnotize DCB1 for a friend.

V+/- are 9.91 and -9.82V.

DC offset with inputs​ shorted is within +/-1 mV.

I'm planning two inputs via a simple DPDT switch but for now it's one in, one out.

I've tried two amps, namely, MyRef Rev C LM3886 chipamp and a TPA3118 PBTL dual mono. The sound with the MyRef is way too dense for my taste. The TPA 3118 sounds balanced, with added weight in the bass and midrange, though upper highs sound a bit recessed (compared to the Kuartlotron buffer).

Current is circa 600 mA with 10//5 Ohms.

Will complete internal wiring with my favourite Helusound AES DMX cable.
 
What is your pot value and type?

The pot is a 20K stepped log from here:
21 Step High Precision Step Potentiometer A20KX2

Any new findings on how it better blends in your given system's tonality?

I handed over the DCB1 to my friend. He is over the moon already:) It convincingly beats his preamp, especially in resolution and "3D-ness". My only regret is I should have completed this earlier (I started soldering The Teabag kit way back in Sep 2014).