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Old 18th October 2006, 11:23 PM   #1
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Default Toccata Phono Preamp

I'm beggining a new project, my own version of Thorsen Loesch Toccata Tube Phono Preamp. There is two versions of this preamp, the first version and the Mrk II.

They can be see at:

http://www.vt52.com/diy/diypage/loesch/toccata.html and
http://www.fortunecity.com/rivendell...a/Toccata.html

I will use the Mk II HV and heater supplies as first supply section.
Then I'll use HV shunt regulator for the preamp HV supplies in dual mono configuration. The heater power supply will feed separate tube heater current regulators. It seems that heater current supply sound better when using DC, a must for a phono preamp with high gain.

The preamp section is from the Toccata first version because the ECC88 is easier to source that the tubes used into the MK II.

I'm keeping the last preamp section to be able to drive more easily the interconnect and the following preamp, or I may use this last section to provide a High/Low gain switch.

My preamp schematic with notes and voltages readings is here:
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File Type: zip toccata preamp.zip (36.4 KB, 245 views)
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Old 18th October 2006, 11:34 PM   #2
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I already aseembled point-by-point the two channels. Each preamp is build on a aluminium L-channel, the 3 stages side by side. The input is at one end, the output at the other end to try to prevent feedback and oscillation. The image is showing that the assembly is quite compact. The signal connections are very shorts. In fact most connections are done only by the parts legs. A metal plate will mount on the top of the preamp L-assy (not shown) and will add some isolation and protection.


I tested the first channel and it is working perfectly. I indicated the differents voltage points on the schematic.

The RIAA curve is rather close, here the results:

RIAA Curve (Actual)
Hz Vout (dB)Theoric
20 +18.7 +19.6
50 +17.0 +17.0
100 +13.4 +13.1
200 +8.5 +8.2
500 +3.0 +2.6
1K 0 (Ref) 0
2K -2.6 -2.7
5K -7.9 -8.1
10K -10.4 -13.6
20K -20.0 -19.4
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File Type: jpg toccata_preamp_small.jpg (46.2 KB, 990 views)
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Old 18th October 2006, 11:41 PM   #3
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As I said, I'm using P-t-P assembly. Circuit junctions are connected to insulated standoffs. The HTR current regulators are using small PCB. I'm also using rather good quality parts: PRP (RIAA & Coupling), Vishay RN60D and Mills resistors, Solen Film (Supply), NOS Vitamin-Q in oil (Coupling) and Rohm Polystyrene (RIAA) caps, gold plated ceramic tube sockets and NOS Amperex ECC88 tubes.

Here a close up of the first stage V1
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File Type: jpg toccata_preamp_v1_small.jpg (49.3 KB, 847 views)
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Old 18th October 2006, 11:45 PM   #4
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The gain for the first section can be adjusted using resistors. The base gain is 4dB. I didn't try the others yet.

The preamp is using a split passive RIAA correction. The second stage V2 has 30dB of gain at 1Khz.

As you probably noted, this preamp is using for the last two stages a battery DC bias. I'm using 3V Lithium small batteries that should last years. They can be easily replace by unsoldering only one resistor in front of the battery socket.
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File Type: jpg toccata_preamp_v2_small.jpg (56.9 KB, 818 views)
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Old 18th October 2006, 11:47 PM   #5
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The last stage can be use as a normal preamp stage. I keep it for know. It has a gain of 28dB and can drive a load of up to 1.2K with a large voltage swing.

This assembly also keep all the parts away from the heat of the tubes, in particular the lithium batteries. All the resistors stay warm to the touch. By suppling the heater regulators with 10V, they also stay warm even with these small heatsinks.

I may need to reduce this last stage ouput to use it with a normal solid state preamp. I'll see...

I was forgetting, these images are for one channel only. Both channel are assembled in a mirror image fashion.
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File Type: jpg toccata_preamp_v3_small.jpg (49.9 KB, 843 views)
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Old 19th October 2006, 12:09 AM   #6
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ugly!
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Old 19th October 2006, 01:43 AM   #7
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New form of construction - "angle iron"??

Looks clean... wonder what your hum floor ends up looking like?

I was able to dispense with the batt bias in a similar design... tube ran a little hotter, but not so hot that it mattered, and I got rid of the issue that the batt bias brings...

Is the ECC88 = 6DJ8?? I get the Euro numbers confused easily...
If so, I'd opt for some other tube myself... as I feel they sound a bit hardish and strident. Others my not agree, ymmv.

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Old 19th October 2006, 02:07 AM   #8
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In fact, I find it not that ugly. It is rather beautifull in an industrial kind of way. Anyway, beauty is a very personnal matter I was inspired by the Audio Experience Concerto Phono preamp for this assembly technic

I used this setup because it is rather easy to modify. I may try some other parts type, in particular the coupling caps.

I'm testing the preamp with a lab power supply and a signal generator for now. Then I'll build the normal power supply. I'm planning to use this preamp at the low gain setting with a step-up transformer.

If the noise floor is too high, I will try some other assembly, possibly a nice PCB with ground plane. This is also one reason why I try not to cut the parts legs too much. I can then re-use them in some other layout.

I cannot comment on the sound since I want to try it with the real supply. Thorsen had some good comments. I will see.

Any more comment on the battery bias? How did you modified your circuit to remove the battery, any schematic?

Here an example of the Concerto preamp

Thanks...
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File Type: jpg audio experience concerto phono preamp_2.jpg (33.7 KB, 835 views)
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Old 19th October 2006, 07:51 AM   #9
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Quote:
Originally posted by Algar_emi
The RIAA curve is rather close, here the results:

RIAA Curve (Actual)
Hz Vout (dB)Theoric
20 +18.7 +19.6
50 +17.0 +17.0
100 +13.4 +13.1
200 +8.5 +8.2
500 +3.0 +2.6
1K 0 (Ref) 0
2K -2.6 -2.7
5K -7.9 -8.1
10K -10.4 -13.6
20K -20.0 -19.4
According to your figures, you have a large error at 10kHz. This may be because by changing the input stage, the RIAA network is no longer being driven by the source resistance the designer intended.
Attached Images
File Type: gif riaa error.gif (9.9 KB, 618 views)
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Old 19th October 2006, 08:50 AM   #10
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Hi Algar, nice and cute work! (in choky's / zen audio's words - ugly)

Can you tell me how to measure the RIAA? Is that just a signal generator and scope?

Cheers!
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