Salas hotrodded blue DCB1 build

[mbain]

DCB1 and Amp Camp

That is a 10K input amp, yes? For driving it as an impedance its good. But I see 17dB gain in that amp which means it won't help go much loud without extra gain in the preamp if the source isn't strong and the speakers aren't sensitive. Any buffer is a unity gain stage. Will play averagely loud but will not bring it to full with a phono as source for example. With a DAC, OK.

Constructing the Amp Camp circuit results in a power amplifier which has 14 dB of voltage gain and 5 watts of output. The input impedance is 10 Kohm, and the damping factor is about 3.
The output noise with the switching supply comes in around 100 uV.

 

[Salas]

Even quieter gain then, like the F5. Isn't the Amp Camp #1 that has a 68K resistor looping from its output to a 10K resistor in series with the input?

 

[mbain]

Even quieter gain then, like the F5. Isn't the Amp Camp #1 that has a 68K resistor looping from its output to a 10K resistor in series with the input?

Yes, I believe it is.

 

[Salas]

Alright, so you would need a 2V RMS DAC or CDP to get 5W+peaks out of it the way music is recorded. And 95dB/W+ speakers to feel dynamic sound in an average size living room.

 

[BigE]

Thanks, I may just try the inductor. It seems that the cheapo resistors are opening up.... will have to measure them voltages again. perhaps later next week. I have learned to leave things a while between changes. It seems to work well

 

[AndrewT]

Andrew,
So sources such as cd/dvd which are high level and can drive many amps directly have no need of a buffer. These sources would merely need an attenuator to reduce volume to a tolerable level. Low level sources like phono will make use of the buffer?
Overall, it's the craziest thing, but I'm not certain I know what I built or how to use it properly. Please explain so even a non audio person can understand.

No.

All Sources can drive, or not drive, the cables connecting to the next stage.

That cable driving capability is principally determined by the current output of the Source.
Active Sources generally have sufficient current to drive domestic length cables.

It's passive outputs that can struggle to drive the capacitance of cables.

It's passive outputs that generally need a buffer to drive the cable.
A volume control is usually a passive potentiometer. These do not have sufficient current capability.

If the pot is attached to the next active stage with only short, low capacitance traces, then it may not need a buffer.

But any normal vol pot of greater than 20k will generally benefit from using a buffer to drive interconnect cables.

 

[BigE]

What load should I use for Duncan amp tools?

Using 10 ohms and a 15V secondary gives a voltage across the load of 18 V, which is sufficient for the regulator.

But using 3 ohms gives around 12-14 V, which is not. This indicates that hotrodding implies that the transformer secondary voltage must also rise!

I do have an 18-0-18 V transformer at 6 amps that I can use instead of the smaller toroid..... Is this advisable? Duncan amp tools suggests that by changing ONLY the transformer, and using 3 ohms for the load, the pre-regulated voltage will now oscillate between 18 and 22 volts. Is that acceptable or is 22 volts too high?

 

[Erlend Sæterdal]

Its a typical implementation. From input RCA to pot, to DCB1 in. You use the DCB1 as a pot buffer for all of your sources. You will need a switch before the pot if you have more than one source. Any relevant configuration copied from switching sources and wiring a pot in front of say a valve preamp will do. We normally use up to 25K pot for faster rise time with the JFET type found in the DCB1.

I need higher then 25 kohm because it is a buffer for LL 1684. Dynamic behaviour is killed into 25 kohm.

 

[BigE]

Why not try another buffer in front of the volume control?

 

[AndrewT]

I need higher then 25 kohm because it is a buffer for LL 1684. Dynamic behaviour is killed into 25 kohm.

use 50k or 100k.
Use RF filtering on every input.

 

[Corpius]

Its a typical implementation. From input RCA to pot, to DCB1 in. You use the DCB1 as a pot buffer for all of your sources. You will need a switch before the pot if you have more than one source. Any relevant configuration copied from switching sources and wiring a pot in front of say a valve preamp will do. We normally use up to 25K pot for faster rise time with the JFET type found in the DCB1.

That's exactly how I did it. I designed my own Volume + input selection board. I used the 25K pot as advised. The relays for input switching are Omron G6K-2P small signal types. I switch the relays using a AtTiny2023 microprocessor and modified 3 pole 4-way Lorlin switch. You can read the whole story here

An externally hosted image should be here but it was not working when we last tested it.

 

[Salas]

I need higher then 25 kohm because it is a buffer for LL 1684. Dynamic behaviour is killed into 25 kohm.

Of course if there is a source compatibility strong reason to use higher value, then the priority is there. When not, we stay low as recommended for that extra definition touch. Which is swamped in heavier deficiencies if the source needs otherwise. You are talking about that build you were combining and lacked dynamically in the initial configuration as a system I suppose. Sorted out completely by now?

 

[Erlend Sæterdal]

Of course if there is a source compatibility strong reason to use higher value, then the priority is there. When not, we stay low as recommended for that extra definition touch. Which is swamped in heavier deficiencies if the source needs otherwise. You are talking about that build you were combining and lacked dynamically in the initial configuration as a system I suppose. Sorted out completely by now?

A Ess 9018 dac with LL 1684 into your buffer is almost perfect. Bass is a bit weak.

 

[Salas]

Running in hot rod also?

 

[Salas]

Why not try another buffer in front of the volume control?

Too many electronics when the source is capable to a pot but still a valid configuration to make a buffer sandwich if there is justification.

 

[Erlend Sæterdal]

Running in hot rod also?

400 ma I think. I will start building a new one next week.

 

[BigE]

What is the maximum pre-regulated voltage that I should use? My transformer has nominal 18V secondaries, but is 110V. I'm in 120 land, so that raises this to 19.6. Duncan PSU designer says the preregulated voltage is 21.3 volts +/- 2 volts, using a 3 ohm load.

Add to that, I usually have 125 or so at the wall plug so there are times when the pre-reg will emit 24 volts. Is this a problem?

 

[Salas]

7-10V DCin-DCout for the regs is OK. There is no problem with maxDCin other than heat.

 

[AndrewT]

My DCB1 run with PSU @ ~+-21Vdc
That gives a drop across the CCS of ~11V. No problems.

Because I design to survive all worst case operation, including max mains supply voltage with output shorted.

 

[mbain]

I am considering use of the Goldpoint 25k stereo stepped attenuator for my volume control. Planning to go from RCA input directly to attenuator then to DCB1. Any suggestions, either positive or negative?