SSR03 - 24V New Build - some newbie questions

[petkovicg]

heaveno there,

I intend to build 24V version of SSR03 to power RIAA (up to 80db gain) phono preamp. I'd like to build best quality, no compromise power supply possible so here are my questions:

1. what secondary voltage transformer do I need? I guess 2x24V will do.
2. phono preamp draws approx. 25mA per rail (DACT CT100). Would 10VA transformer be an overkill?
3. what is the benefit of installing parallel rectifier diodes 1N5404G  as shown on the schematics?
4. what type 5W resistors should be? are wirewound types OK?
5. does LT1028 produce better results than AD825
6. due to relatively low current draw of 25mA can I use smaller heatsinks (14 C/W). if so, I coud install two units one on top of each other in 2U enclosure

http://export.farnell.com/jsp/search/productdetail.jsp?sku=1892326

Many thanks for your input,
Goran

[peranders]

I'll come back on this next week.

[tk-don]

Maybe I can be of a little help in the meantime without wreaking too much havok. :-)

1) 2x24VAC (in parallel) is fine. The VAC rating is RMS for all toroids I've seen. 22VAC is also fine.
You''ll need 4.5V to 5V more on the input compared to the final output voltage to allow for voltage drops in the PSU (the LM317 and output transistor) + the voltage drop of 1.4V in the full wave rectifier. The DC input voltage you'll need is thus 1.4V+5V+24V=30.4VDC=21.5VAC.

2) Yes, something like 4 VA would be fine, but typically you won't save money in that segment. 4VA and 10VA should be nearly evenly priced.

3) From what I understand the schematics is a gross schematic showing all possible configurations of the PCB with the LM431. The only benefit of the 1N5404 is higher current draw whereas the 1N4007 is rated to 1A.

4) If they fit, then why not. Some 5W resistors will not within the available space, though.
The information and calculations I provide here are likely not be correct: But the resistors provide a compromise of inrush current limitation and voltage drop. The toroid will provide minimum 33.84VDC when fully loaded. The output should be 24VDC and components causes a loss of about 6V including 1 diode of the rectifier. The voltage drop for each resistor would then be 3.84V/2 in best case and with the low current requirement you have, 0.6W resistors may just work fine. But I think I'm overlooking something, since the inrush current of the capacitor bank would demand a high power rating of the resistors. P-A can answer this along with the other questions, I'm sure.

I'll be building a 15V SSR03 using 3W 1.2 ohms resistors before the rectifier, but to be honest I haven't considered that value and power rating much...

[peranders]

heaveno there,

I intend to build 24V version of SSR03 to power RIAA (up to 80db gain) phono preamp. I'd like to build best quality, no compromise power supply possible so here are my questions:

1. what secondary voltage transformer do I need? I guess 2x24V will do.
2. phono preamp draws approx. 25mA per rail (DACT CT100). Would 10VA transformer be an overkill?
3. what is the benefit of installing parallel rectifier diodes 1N5404G  as shown on the schematics?
4. what type 5W resistors should be? are wirewound types OK?
5. does LT1028 produce better results than AD825
6. due to relatively low current draw of 25mA can I use smaller heatsinks (14 C/W). if so, I coud install two units one on top of each other in 2U enclosure

http://export.farnell.com/jsp/search/productdetail.jsp?sku=1892326

Many thanks for your input,
Goran

1 Yes

2 No

3 You should use one of them, not both. For small currents, use 1 amp diodes.

4 Use metal film since you will draw out so small current

5 Slightly better in terms of lower noise and lower output impedance. The AD825 has very good phase characteristics which is good for stability.

6 If it's only about 25 mA (or 50 mA) you won't need any big heatsink, maybe none.

[petkovicg]

Thanks so much for your advises. I have one more question if you don't mind.

Since all four SSR03's will be installed in the same enclosure I will need four transformers as well. Is there any reason why I shouldn't install two 20VA toroidal transformers one on top of the other? My enclosure is about 7.5 cm high and transformers are about 2.5 cm high so I could have about 2 cm clearance between top and bottom transformer. Both transformers will be held in place by M6 nylon bolt running through middle of "donuts". And I will have another stacked pair next to these about 3cm apart.

Your comments are highly appreciated.

Cheers,
Goran

[peranders]

You can mount the transformers tightly together but I wouldn't use a nylon bolt. The shape of the nylon is maybe not stable over time. Use a steel bolt.