Elliot's RIAA Preamp questions

[Paapaa]

Some questions regarding Rod Elliot's RIAA Preamp. Their forum is a bit quiet so:

1. What is the estimated power consumption of the circuit in normal usage with +/- 15V power supply?

2. How necessary it is to bypass the op-amp supply line if used with 2 9V batteries?

3. If op-amps are powered with Richie00boy's power supply is it enough to bypass with 100nF capacitors at each line without the electrolytics? I'm trying to make as small PCB as possible.

4. I have read about star grounding. Is it needed here if GND lines on PCB are made wide enough and distances small enough. I guess the currents are so small that there should be very minimal voltage drops to cause anything noticeable? (My principle is: if you can't hear it in ABX blind test it doesn't exist)

5. The PCB photo on the project page has some additional components not in the schematics. There are 16 capacitors in the picture and only 2 x 5 (=10) in the schematics. If you add 2x2 capacitors for bypassing there are still 2 capacitors more in the picture. So what might be the additional capacitors in the picture?

Thanks in advance!

(Sorry if this is on a wrong forum...)

[Paapaa]

And one more:

6. What is considered an acceptable ripple voltage when powering this preamp?

[Piercarlo]

Originally posted by Paapaa
Some questions regarding Rod Elliot's RIAA Preamp. Their forum is a bit quiet so:

1. What is the estimated power consumption of the circuit in normal usage with +/- 15V power supply?

This is a preamplifier conceptually very similar to one i've build a ten-twelve years ago and, once provided a good filtered supply, is uncritical and well performant. Estimated consumption (without signal) is about 5 mA for each op-amp.

2. How necessary it is to bypass the op-amp supply line if used with 2 9V batteries?

Is still necessary. Ripple is not the only nuisance to be filtered on power supplies.

3. If op-amps are powered with Richie00boy's power supply is it enough to bypass with 100nF capacitors at each line without the electrolytics? I'm trying to make as small PCB as possible.

If the preamp is long-wire connected to the PSU my advice is of using electrolytics non either side of the cable: at the outlet of PSU and at the inlet of preamp.

4. I have read about star grounding. Is it needed here if GND lines on PCB are made wide enough and distances small enough. I guess the currents are so small that there should be very minimal voltage drops to cause anything noticeable? (My principle is: if you can't hear it in ABX blind test it doesn't exist)

A couple of month ago i've build for a friend a phono preamp very similar to that proposed by you. All the connection to ground (in, out and feedback) were routed to a single pad of a breadboard PCB. The key issue here, where don't circulate very important currents (op-amps draw forth and back their current only on the supply rails, not on ground), is granting the same "zero voltage reference" to all the part of circuit that must be refererred to: input network, feedback network and output return. Chassis must be mounted as a pure shield, connected to the same previous ground point and *absolutely not used for ground interconnections*; this mean what input and output ground connection of RCA pin should be insulated from the chassis and connected to ground by their own separate path.

About ripple rejection: a ripple of maximum 1 mV (counting in also the induced ripple by wandering magnetic fields due to transformers near the circuit, also those of others appliances) is recommended. Less is better however...

Hi
Piercarlo

[Paapaa]

Originally posted by Piercarlo

If the preamp is long-wire connected to the PSU my advice is of using electrolytics non either side of the cable: at the outlet of PSU and at the inlet of preamp.

The PSU will be very close (10cm or so) to preamp and it has, I believe, proper filtration as you can see in the PSU schematics: 100n + 47u capacitors at output side. I tried to search for some guidance on bypassing but all (at least OPA2132) datasheets suggests only single bypass capacitor and that is usually 10nF.

So is it safe to assume that if I have a quality PSU not too far away a single 100nF bypass cap should be adequate to eliminate oscillations?

A couple of month ago i've build for a friend a phono preamp very similar to that proposed by you. All the connection to ground (in, out and feedback) were routed to a single pad of a breadboard PCB. The key issue here, where don't circulate very important currents (op-amps draw forth and back their current only on the supply rails, not on ground), is granting the same "zero voltage reference" to all the part of circuit that must be refererred to: input network, feedback network and output return. Chassis must be mounted as a pure shield, connected to the same previous ground point and *absolutely not used for ground interconnections*; this mean what input and output ground connection of RCA pin should be insulated from the chassis and connected to ground by their own separate path.

Now I have to ask a bit more. These are all the grounds I have in RIAA circuit:

1. L+R input GND
2. L+R output GND
3. PSU GND
4. Turntable earth connector

Now I have them connected to 6 pads which are located next to each other in a row - 0.1" distance between each pad.

The preamp is inside its own plastic box so no connection to any chassis.

[Piercarlo]

Originally posted by Paapaa
[B]

The PSU will be very close (10cm or so) to preamp and it has, I believe, proper filtration as you can see in the PSU schematics: 100n + 47u capacitors at output side. I tried to search for some guidance on bypassing but all (at least OPA2132) datasheets suggests only single bypass capacitor and that is usually 10nF.

So is it safe to assume that if I have a quality PSU not too far away a single 100nF bypass cap should be adequate to eliminate oscillations?

You have to connect a single ceramic capacitor of the mentioned value the closest as possibile to the supply pin of op-amps.

Now I have to ask a bit more. These are all the grounds I have in RIAA circuit:

1. L+R input GND
2. L+R output GND
3. PSU GND
4. Turntable earth connector

Now I have them connected to 6 pads which are located next to each other in a row - 0.1" distance between each pad.

The preamp is inside its own plastic box so no connection to any chassis.

If you have connected all these grounds on the same row the circuit should work well. important is how the ground PSU is routed; if center ground of PSU capacitors is separated from the other ground is better. To the same center capacitor ground you may connect the Turntable earth connector (provided that on the turntable this connection *don't share anything with other signal grounds*; if not so is best to left open this connection). However the best thing is hearing how much hum is again audibile; if this hum, with maximum volume, is reasonably covered by the white noise of the input (with the MM cartridge in circuit - don't hope in a pre-amp as so good to exhibit only white noise without hum residuals... is very, very difficult to obtain it and surely is not a goal reachable at all with plastic enclosures!), then you can stay quite still: once you play disc the rumble is well consistent and usually swamp other noisy contributors.

Hi
Piercarlo