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Old 23rd January 2013, 03:59 PM   #1
Loren42 is offline Loren42  United States
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Default Preamp Design Review Request

Greetings,

After getting this circuit from Eli (thank you!), I decided to try to roll a preamp design based on this circuit (see below).

I was hoping to get some feedback before I cut the PCB. My main concern is component selection since this impacts layout. More on that shortly.

First, here is a link to the system architecture.

http://www.mdbq.net/audio/sysblk01.pdf

The two power supply modules are simply linear regulated supplies made by International Power. They are cheaper than rolling my own when you consider the cost of making a PCB and sourcing a transformer.

The phono stage is TBD and is not considered in this review.

The FET line out is Eli's work and is described in another post about driving long lines.

The crux of the review is the line amp. Yes, I plan to have a tone circuit defeat switch, but the tone controls are a desired feature, too. The attenuation for the defeat path is to be determined.

Here is the schematic:

http://www.mdbq.net/audio/LineAmpSchem01.pdf

The resistors are either 1/4 metal film or 2W metal film. The board layout has provisions for the 2W resistors where they might be needed.

The capacitors are a little more tricky. I normally order parts from Digikey for my business, so it is convenient to use them as a source. Here are the Digikey part numbers for the three caps:

C1 & C4 1800 pF 500 WVDC Mica 338-2871-ND
C2 & C3 10 F @ 100 WVDC Polyester, Metallized Film 495-4099-ND
C5 & C6 47 F @ 200 WVDC Electrolytic P5914-ND

DigiKey Electronics - Electronic Components Distributor

The filaments are tapped right at the sockets. Pins 4, 5, & 9 are brought out, but I plan on using only pins 4 & 5 powered by 12 VDC.

The board is a 4-layer board. One internal layer is used for ground. The second internal layer is not used.

I did not think using an internal layer for 250 VDC was wise. I am not sure what to do with this layer.

One thought is to delete it.

Another idea is to simply place a grounding via at only one corner of the board to this plane (so there is no ground loop), but it would create a capacitor,

What is the best thing to do?

The layout can be found at:

http://www.mdbq.net/audio/PCB01.jpg

Please let me know what you guys think and what is right or wrong. I appreciate your feedback and will be happy to answer any questions.
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Old 23rd January 2013, 04:41 PM   #2
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In my opinion:
1) the switch to bypass the tone stage is useless.
2) Use of a electrolytic capacitor of too large voltage rating is also useless, it is said that a capacitor of, say, 100V WV used with a voltage of 16V never will be formed as it must, and the results is a lower final cap. I don't know if it is true or not.

Lots of luck in the project!.

Regards,
Osvaldo.
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Old 23rd January 2013, 04:49 PM   #3
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Osvaldo de Banfield View Post
In my opinion:
1) the switch to bypass the tone stage is useless.
2) Use of a electrolytic capacitor of too large voltage rating is also useless, it is said that a capacitor of, say, 100V WV used with a voltage of 16V never will be formed as it must, and the results is a lower final cap. I don't know if it is true or not.

Lots of luck in the project!.

Regards,
Osvaldo.
Thanks for the reply. What voltage should the cap be rated for?
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Old 23rd January 2013, 05:14 PM   #4
TheGimp is offline TheGimp  United States
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C5,C6, try for 10V, 6.3 if you can find it.
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Old 23rd January 2013, 05:31 PM   #5
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by TheGimp View Post
C5,C6, try for 10V, 6.3 if you can find it.
Okay:

Digikey # 493-4636-1-ND

UKA1A470MDD1TD Nichicon | 493-4636-1-ND | DigiKey
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Old 23rd January 2013, 05:34 PM   #6
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A WV immediately upper than the expected in the circuit. Example, in a 12V circuit, it is of no sense to use 200V cap's, use, 16 or almost 25V. You will save cost, space, and hours of life of them.

I don't know about cap manufacturers for audio, and their reputation and how them "sound". Positively I can affirm that Nichicon are one of the bests for switching power supplies. Lots of experience in this job.
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Last edited by Osvaldo de Banfield; 23rd January 2013 at 05:40 PM. Reason: More explain
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Old 23rd January 2013, 07:22 PM   #7
tomchr is offline tomchr  United States
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I'd scrap the tone control. If you want to correct for room acoustics or recording imperfections, I suggest using a graphic EQ instead.

I'd just go with the right part of the circuit. So from W5 and on.

It's not clear from your layout, but it looks like you're planning to use a ground plane. I recommend against that. You have better control over the ground currents if you use a star ground topology. I also don't like ground planes in high-voltage circuits as the clearance requirements between HV connections and the ground planes make the ground plane really spotty and rather useless.

You'll need grid leak resistors to ground and a volume control somewhere. Grid stoppers would be handy as well.

If I interpret your schematic correctly, the line stage and output stage are a grounded cathode amp followed by a cathode follower. The LF pole on the cathode follower is set by C1, R7. 1.8 nF, 1 Mohm makes a pole at 90 Hz. So your amp will cut off most of the LF. I suggest increasing C1, C4 to at least 22 nF. Higher if you want to avoid excessive phase change in the lower octaves of the audible frequency range.

~Tom
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Last edited by tomchr; 23rd January 2013 at 07:28 PM.
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Old 23rd January 2013, 08:48 PM   #8
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1st and foremost, you switch the entire tone control block, not just the signal shaping elements, into or out of the signal path.

Max Robinson's overall "unity gain" Baxendal implementation is merely the point of departure. Remember, I said 4 bottles just for tone controls is too much. Replace the cap. coupled cathode followers with DC coupled ZVN0545A source followers and you halve the bottle count. You also reduce the number of caps. in the signal path, which is "always" a good thing. The tweaked RCA phono preamp schematic I've uploaded shows how to implement DC coupled ZVN0545A source followers.

Please no polyester film in the signal path. "Teflon", polystyrene, and polypropylene constitutes the sonic transparency pecking order. For several practical reasons, polypropylene film is (IMO) usually the "best" choice. C1 and C4 disappear, when DC coupling is employed. Use Nichicon KZ series 'lytics for C5 and C6.

The value of O/P coupling caps., like C2 and C3, is controlled by the I/P impedance of the downstream circuitry. The coupling cap./I/P impedance combo form a high pass pole. Most of the time, F3 of that pole should be no greater than 5 Hz. Metalized polypropylene Solens bypassed by either 716P series "Orange Drops" or, for those with a minor anal streak, MultiCap PPFX parts will work well.

Use the Sovtek 12AX7LPS, a genuine 7025 equivalent, in the tone control circuitry and AC heating is fine. The 'LPS employs a hum bucking/spiral wound heater. Between hum bucking and biasing off B+, it will be quiet. Save the DC heaters for the phono section.

100 Ω carbon composition stoppers on the 'X7 grids is a reasonable precaution. Please observe that the phono preamp takes that precaution.
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File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 143 views)
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Old 23rd January 2013, 08:58 PM   #9
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Eli Duttman View Post
1st and foremost, you switch the entire tone control block, not just the signal shaping elements, into or out of the signal path.

Max Robinson's overall "unity gain" Baxendal implementation is merely the point of departure. Remember, I said 4 bottles just for tone controls is too much. Replace the cap. coupled cathode followers with DC coupled ZVN0545A source followers and you halve the bottle count. You also reduce the number of caps. in the signal path, which is "always" a good thing. The tweaked RCA phono preamp schematic I've uploaded shows how to implement DC coupled ZVN0545A source followers.

Please no polyester film in the signal path. "Teflon", polystyrene, and polypropylene constitutes the sonic transparency pecking order. For several practical reasons, polypropylene film is (IMO) usually the "best" choice. C1 and C4 disappear, when DC coupling is employed. Use Nichicon KZ series 'lytics for C5 and C6.

The value of O/P coupling caps., like C2 and C3, is controlled by the I/P impedance of the downstream circuitry. The coupling cap./I/P impedance combo form a high pass pole. Most of the time, F3 of that pole should be no greater than 5 Hz. Metalized polypropylene Solens bypassed by either 716P series "Orange Drops" or, for those with a minor anal streak, MultiCap PPFX parts will work well.

Use the Sovtek 12AX7LPS, a genuine 7025 equivalent, in the tone control circuitry and AC heating is fine. The 'LPS employs a hum bucking/spiral wound heater. Between hum bucking and biasing off B+, it will be quiet. Save the DC heaters for the phono section.

100 Ω carbon composition stoppers on the 'X7 grids is a reasonable precaution. Please observe that the phono preamp takes that precaution.
I have 1.7 Amps at 12 VDC available for heaters, so I should have plenty of reserve for all tubes.
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Old 24th January 2013, 01:01 AM   #10
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Even with plentiful 12 VDC, switching to DC coupled ZVN0545A source followers is advisable. Fewer caps. in the signal path and less waste heat simply can't be bad.
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