C3g Aikido Phono

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Nick
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#46 Re: C3g Aikido Phono

Post by Nick »

Worth saying that early versions of LCR's I built had the same problem I am describing here. Pentodes are certainly better at low loads after all but I eventually went with cathode follower with feedback into the LCR. And yes pentodes are more stable.
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#47 Re: C3g Aikido Phono

Post by IslandPink »

Andrew wrote: Mon Jan 30, 2023 5:57 pm
Nick wrote: Mon Jan 30, 2023 5:48 pm But as mark did you can reduce the minimum load by keeping the 5k6 anode load but including a series r in the feed to the riaa.
Err, I'm a bit rusty, so I would need to think that thru'!

I wonder, out aloud, if you might need to increase the anode load on the pentode if you did that? The pentode load matches the impedance looking into the network. Wouldn't it now need to match the impedance looking into the series resistor plus the network?
You do seem a bit rusty Andrew !
Let's say you scale up the RIAA to about 20 or 22k effective. This allows the first RIAA cap to be 0.1uF.
The 'R1' input resistor ( as per all-in-one RIAA nomenclature ) is now around 22k.
Of course, for any finite drive impedance, this is included in the calc. So for mine I have about 2k Zout from my D3a, then 20k resistor.
In Simon's case, he would probably stick with the 5k6 anode load on the C3g ( for gain reasons ) then add an in-line resistor of about 15 k or 16k.

As for the coupling cap location, one advantage of having it after the RIAA is that the RIAA caps are then biased up by a signifcant voltage which ought to make them less prone to noise caused by any loose layer winding and voltages either side of zero. I don't know if anyone ever did any measurements on that, but it's the way I'v always done it.
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#48 Re: C3g Aikido Phono

Post by Nick »

Yes scale the network to match.
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#49 Re: C3g Aikido Phono

Post by simon »

IslandPink wrote: Mon Jan 30, 2023 6:14 pm Let's say you scale up the RIAA to about 20 or 22k effective. This allows the first RIAA cap to be 0.1uF.
The 'R1' input resistor ( as per all-in-one RIAA nomenclature ) is now around 22k.
Of course, for any finite drive impedance, this is included in the calc. So for mine I have about 2k Zout from my D3a, then 20k resistor.
In Simon's case, he would probably stick with the 5k6 anode load on the C3g ( for gain reasons ) then add an in-line resistor of about 15 k or 16k.
So, add a 15k or 16k resistor before the RIAA, change the 470nF and 47nF caps for 100nf and 10nF, and the 680R and 68R resistors for 3200R and 320R? And the 1068R for 5020R? Is it that simple?
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#50 Re: C3g Aikido Phono

Post by IslandPink »

I think so yes. Maybe Andrew can comment on one thing - is the 1068R in your circuit a little trick to make the second RIAA cap a stock value ?
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#51 Re: C3g Aikido Phono

Post by Andrew »

IslandPink wrote: Mon Jan 30, 2023 6:14 pm
Andrew wrote: Mon Jan 30, 2023 5:57 pm
Nick wrote: Mon Jan 30, 2023 5:48 pm But as mark did you can reduce the minimum load by keeping the 5k6 anode load but including a series r in the feed to the riaa.
Err, I'm a bit rusty, so I would need to think that thru'!

I wonder, out aloud, if you might need to increase the anode load on the pentode if you did that? The pentode load matches the impedance looking into the network. Wouldn't it now need to match the impedance looking into the series resistor plus the network?
You do seem a bit rusty Andrew !
Let's say you scale up the RIAA to about 20 or 22k effective. This allows the first RIAA cap to be 0.1uF.
The 'R1' input resistor ( as per all-in-one RIAA nomenclature ) is now around 22k.
Of course, for any finite drive impedance, this is included in the calc. So for mine I have about 2k Zout from my D3a, then 20k resistor.
In Simon's case, he would probably stick with the 5k6 anode load on the C3g ( for gain reasons ) then add an in-line resistor of about 15 k or 16k.

As for the coupling cap location, one advantage of having it after the RIAA is that the RIAA caps are then biased up by a signifcant voltage which ought to make them less prone to noise caused by any loose layer winding and voltages either side of zero. I don't know if anyone ever did any measurements on that, but it's the way I'v always done it.
Nick said what I thought was the case, or perhaps what I didn't describe very well, you need to scale the network to match :)

The network, including any additional series resistance, needs to match the driving load, or the terminating load.
Last edited by Andrew on Mon Jan 30, 2023 8:31 pm, edited 3 times in total.
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#52 Re: C3g Aikido Phono

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IslandPink wrote: Mon Jan 30, 2023 7:49 pm I think so yes. Maybe Andrew can comment on one thing - is the 1068R in your circuit a little trick to make the second RIAA cap a stock value ?
Yes, it's all stock values and if I recall the 68R in series with the 1k adds a touch of nice bass lift, which compensates for speaker roll off and other losses

I shall have to get the Spice out.
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#53 Re: C3g Aikido Phono

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simon wrote: Mon Jan 30, 2023 6:37 pm So, add a 15k or 16k resistor before the RIAA, change the 470nF and 47nF caps for 100nf and 10nF, and the 680R and 68R resistors for 3200R and 320R? And the 1068R for 5020R? Is it that simple?
Yes, kind of, but your'e better getting Spice out, to double check. Don't make life harder than it has to be.
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#54 Re: C3g Aikido Phono

Post by simon »

I'd have to learn Spice first sadly
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#55 Re: C3g Aikido Phono

Post by Nick »

The network, including any additional series resistance, needs to match the driving load, or the terminating load.
Why, we are not talking about a transmission line. You are still thinking about LCR's. Just make the anode resistor right for the valve, 5k6 seems fine, then make the load > 10 times the source impedance and forget about it after that.

And even for a LCR they only need to be either driven or terminated by their characteristic impedance and thats easy with a resistor on the end and a low impedance driver. Trying to match the driver to the LCR sounds like a good idea in principle, but harder than a fixed value resistor.
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#56 Re: C3g Aikido Phono

Post by Nick »

As for the coupling cap location, one advantage of having it after the RIAA is that the RIAA caps are then biased up by a signifcant voltage which ought to make them less prone to noise caused by any loose layer winding and voltages either side of zero. I don't know if anyone ever did any measurements on that, but it's the way I'v always done it.
Sounds like a good idea, but IMHO is a solution in search of a problem. I would just use polystyrene instead of film and foil and not bother as they are potted. I just prefer the relative cheapness that the RS polystyrene 63v ones give, and the fact they are 1% tolerance and cheap enough to buy 10 and select the best pair for the price of a pair of caps from HiFi Collective.

Also, any microphonics in the cap is likely to be swamped by the same in the valve.

For me the accuracy and even more important the channel matching of the RIAA is above any other consideration. Save the good film caps for the coupling positions.

But I have become cynical.
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#57 Re: C3g Aikido Phono

Post by IslandPink »

Yes, maybe so !
I tried polystyrene ( 2 types ) in the upper RIAA turn and didn't like them that much. Maybe it's my fault for using ribbon tweeters.
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#58 Re: C3g Aikido Phono

Post by IslandPink »

Nick wrote: Mon Jan 30, 2023 9:17 pm .... for the price of a pair of caps from HiFi Collective.
Yes, the prices are shocking now though. I'm just trying to order some other things for Jon alongside a pair of ZN's, and the metallised films from Mundorf etc are obscene prices.
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#59 Re: C3g Aikido Phono

Post by simon »

Yeah the price of caps shocked me - they're in the region of triple what they were last time I looked.
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#60 Re: C3g Aikido Phono

Post by simon »

How about a nice matched quad of these?
https://www.hificollective.co.uk/catalo ... r-max.html
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