AudioSector-chip amp kits, dacs, chassis

[Peter Daniel]

Originally posted by rjkdivin
R7 and R9 are the correct ones to replace....correct Peter? I noticed you added the Caddock-spaced thru hole pads at R8 and R9, but not R7, and it threw me off a little.

Actually R8 and R9 are first to be replaced. Those are I/V resistors and if you check the traces underneath, they both connect with output coupling caps. In some setups using Caddocks here, in place of Rikens, may be beneficial (but may still require choosing different output caps).

Recently it's been brought to my attention that placing Caddock TF020 for R7 also improves things a bit, so you may try it as well (1k5)

All 3 Caddocks will fit in top layer, R7 needs pin properly formed.

[Peter Daniel]

Originally posted by rjkdivin
I'm planning to build two more boards without the PSU section, or at least without the inlet and transformer, and put them into the cases I built for my two NOS DACs, sharing one PSU switched between the two DACs in each case.

It looks like the PCB cut could be made to feed the second board from the first transformer secondary (so the PSU diodes, caps and inductors would be duplicated); or you could cut it right behind the inductors and make the connection at R1,2,3,4 (so none of the PSU would be duplicated). To switch the PSU between the two boards would require a 4PDT switch? Any suggestions?

I made a similar setup, in case both SPDIF and USB DACs are needed (and fed from common supply). I didn't want to go into switching I2S lines, not to compromise the performance.

The rectifiers and C1/C3 are common to both DACs, everything else is separate. The supply connections are done through pair of wires on each side, from the pads sideways to rectifiers (now you know what they were intended for

You may install 2PDT switches on each side to disconnect power from DAC on top.

[jan.didden]

Peter where can one buy the RCA dual input blocks shown in the picture above?

Jan Didden

[Peter Daniel]

You may check with percyaudio.com , he seem to have an item like that in his catalog, although it's not clear if it's dual:

Cardas PC Mount: Ag/Rh plated, right angle or straight… $20.00 pr.

Or you can get it directly from me at $20/pair; I get them from Cardas.

[Frode]

I just powered up my new LM3875 GC, but I get no sound, just hum. With 2x18V secondaries, I get just below 25V on pin 1 and -25V on pin 4 on both chips. Any ideas?

Frode

[Peter Daniel]

You didn't install R1 (220R). Unless you put jumper there, there is no input signal connected.

With 100uf caps only (on amp board) you will be getting some hum. You still need to add bigger caps (like BG STD 1000uf) on rectifiers board (C1 and C2)

[Frode]

Forgot the jumpers!

Frode

[bfslm]

Peter,

Very interesting in LM3875 but have some question before I mail you for order (DAC & amp).
My speaker is only 6ohm, 88Hz Sonus faber bookshelf
My taste of music: Jazz, pop. I'm not are rock fan,

Does LM3875 suitable for my speaker, do you suggest me to brigd/paranel or looking else ?

For DAC i'll contact you via email, for Amp i can order via audio sector, is it?

Many thanks for your advise

[bfslm]

All,

Some concern on transformer need help.

For LM3875, should we use 2 transformer 160 VA mono block or 1 of 300 VA or 2 300 VA transformers, what option extract best performance from LM3875. For 6 ohm speaker, what Dc voltage workbest: 18, 20, 22, or 25 V DC. Becasuse i can customer make for trans. We use 220V AC in our country.

Many thanks

[AndrewT]

Originally posted by bfslm
For LM3875, should we use 2 transformer 160 VA mono block or 1 of 300 VA or 2 300 VA transformers, what option extract best performance from LM3875. For 6 ohm speaker, what Dc voltage workbest: 18, 20, 22, or 25 V DC.

the chip can do upto 60W into your chosen load.
Using the normal factor, this requires a transformer with a VA rating of between 60VA and 120VA.
But small transformers have a high regulation value. This results in a high quiescent voltage making the chip run hotter than necessary and also results in the supply voltage sagging badly on longer duration transisents.
I think your suggestion of 160VA is probably about right for a single channel chipamp (monoblock).
If you can get a slightly bigger transformer cheaply then there is a small benefit in reduced regulation.

If you want two channels, then the transformer should be between 120VA and 240VA. Your 160VA would do just nicely for two channel as well. Two channel is more difficult to get hum free operation.

I recommend a supply voltage between 56Vdc(+-28V) and 70Vdc(+-35V) for 6ohm speaker and preferably at the lower end, particularly since the max output current is 4A to 6A when the chip is cold.

You need to identify your transformer and find out it's regulation, then you can decide which AC voltage you require to give your target DC voltage.