I don't know whether or not the schematic is an accurate representation of the PCB layout. Assuming it is...
That looks like pin 9 is grounded to 0V signal ground. I see no voltage divider for lifting the ground of the heater supply. I think that's referenced to 0V.
The schematic for the heater supply...
...shows a "15V - 15V" winding that is not center tapped. What does that mean? I don't know. I assume it's a 15VAC secondary. Assuming that's correct, D1 will rectify that to about 19V DC. It looks like that's grounded on the positive side, so that means it's a -19V DC supply, with the voltage drop from 10R 5W (R26) subtracted. With three 12A*7 tubes' heaters as the load, that's 0,45A, so 19V - 4.5V = 14.5V DC on the heaters? Maybe there are more losses than that. It could be that the raw voltage is only 17V so the final DC voltage comes out to -12.5V. Let's assume that's correct.
1. The DC heater supply is grounded on one side. No center tap. That will increase the chances of hum from the heater supply. Since it's grounded on the DC side, you do not want to ground it again on the AC side, so you can't put in a voltage divider to balance the secondary.
2. There's not a lot of ripple filtering on the heater supply. 10 ohms * 4700uF, and just one stage of RC after the reservoir cap.
3. It could be that the hum is coming from the heater supply. I've never had good luck with a simple rectified DC supply like that. I either go with AC heaters or a decent regulator (LM317, 7812, LT1085, etc.). Unfortunately, the designers of this product spent their money on a 10R 5W resistor instead of a voltage regulator IC. Wouldn't that have cost about the same?
Maybe increasing the value of C2 would help. It could be 10,000uF 25V. But that might not be enough. I have my doubts, but I certainly don't know for sure.
That looks like pin 9 is grounded to 0V signal ground. I see no voltage divider for lifting the ground of the heater supply. I think that's referenced to 0V.
The schematic for the heater supply...
...shows a "15V - 15V" winding that is not center tapped. What does that mean? I don't know. I assume it's a 15VAC secondary. Assuming that's correct, D1 will rectify that to about 19V DC. It looks like that's grounded on the positive side, so that means it's a -19V DC supply, with the voltage drop from 10R 5W (R26) subtracted. With three 12A*7 tubes' heaters as the load, that's 0,45A, so 19V - 4.5V = 14.5V DC on the heaters? Maybe there are more losses than that. It could be that the raw voltage is only 17V so the final DC voltage comes out to -12.5V. Let's assume that's correct.
1. The DC heater supply is grounded on one side. No center tap. That will increase the chances of hum from the heater supply. Since it's grounded on the DC side, you do not want to ground it again on the AC side, so you can't put in a voltage divider to balance the secondary.
2. There's not a lot of ripple filtering on the heater supply. 10 ohms * 4700uF, and just one stage of RC after the reservoir cap.
3. It could be that the hum is coming from the heater supply. I've never had good luck with a simple rectified DC supply like that. I either go with AC heaters or a decent regulator (LM317, 7812, LT1085, etc.). Unfortunately, the designers of this product spent their money on a 10R 5W resistor instead of a voltage regulator IC. Wouldn't that have cost about the same?
Maybe increasing the value of C2 would help. It could be 10,000uF 25V. But that might not be enough. I have my doubts, but I certainly don't know for sure.
I wondered that with the original EAR834p as well... I use LT1085, but it also takes a bit of space, couple of resistors and a little heat sink. not quite as cheap as the 10R 5W resistor, but it performs much better.
btw - I also thought the hum might be from the heaters...
Wouldn't referencing the heaters above ground be pretty much "de rigeur" for a phono stage? I realize the original 834p doesn't have an elevated heater supply, but also doesn't use a tube rectifier.
jeff
jeff
I can't speak for the OG EAR834p but the clone boards available nowadays do elevate the heater supply 70v or so...
The black and gold boards use an elevated heater and I use a 20ma 20H choke in the main power supply instead of just using a resistor.
I have worked on and listened to one of these "little bear" units and don't remember there being any bad hum once I twisted the internal AC wiring between the transformer and the board.
I have worked on and listened to one of these "little bear" units and don't remember there being any bad hum once I twisted the internal AC wiring between the transformer and the board.
Douk 834p Schematic, elevator circuit in red
E the 3rd valve is a Cathode Follower, 100R on the plate and 68K at the Cathode makes for a higher Kv. HKv of 12ax7 is 200v but only 90v for 12at7, which I use.
E2 measured my Douk build, 117v on the v3 Cathode and heaters elevated 32v.
w
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Hi ejp, I can't seem to find anything you have written discussing Tim de Paravancini's fundamental design. I would just like to learn more.
In any case, active RIAA using feedback is hardly novel. Dynaco did it in the 1950's (for example) right? One thing for certain, pretty much every circuit is not novel when it comes to valves/tubes today. It has all been done before...
Please do advise further if possible. This is a DIY board after all. 🙂
In any case, active RIAA using feedback is hardly novel. Dynaco did it in the 1950's (for example) right? One thing for certain, pretty much every circuit is not novel when it comes to valves/tubes today. It has all been done before...
Please do advise further if possible. This is a DIY board after all. 🙂
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I wrote that it needs a shunt resistor and much more loop gain.
I didn't say anything was novel. What I said was that these designs that rely on the limited open-loop gain can never be accurate, as the transfer function algebra shows.
If you assume infinite open loop gain, set the resistor to 750k, and add a 9M resistor in parallel with the entire feedback network, it is very accurate. But you need at least 80dB open-loop gain to get that.
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Well, some of my hum problems have been solved. I found a faulty ground lift switch and also replacing a bad filter capacitor in the heater circuit dropped the hum enough to actually here the tube noise.
Thank you ejp for your explanation. So failing the 80dB open-loop gain, the effect in your opinion is that the RIAA correction will be unstable?
80dB open-loop gain is seriously quite a lot. The entire amplifier makes approx. 80dB gain total (before RIAA feedback loop). This would suggest a modified Dynaco-style feedback loop (to the cathode of the input 12ax7) might be preferable?
80dB open-loop gain is seriously quite a lot. The entire amplifier makes approx. 80dB gain total (before RIAA feedback loop). This would suggest a modified Dynaco-style feedback loop (to the cathode of the input 12ax7) might be preferable?
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That's probably getting to be where these normally work. Maybe see if the 7025 lowers the tube noise now?
Cheap parts & lack of QA are probably 90% of why people have problems with these built in China preamps.
jeff
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I had a quick look on the Nobsound site. There's absolutely no mention of warranty coverage.
jeff