Dynavox TPR-2 preamp measurements

[dougi]

INTRODUCTION AND DESCRIPTION

Hi all. I'm posting measurements of the Dynavox TPR-2 line/phono tube output preamp. Sorry US members, only available in 230V AC.

Pics, description and specs can be found on the Audiophonics page. It's a solid state phono preamp with a 12au7 triode output, used direct for the line input. With a seperate power supply unit to generate the various AC voltages, which are then rectified within the main unit.

A circuit seems to be floating around for it on ye olde flickr. As can be seen from the circuit:

• two stage active RIAA for phono, with supposedly LT1028 in the first stage, NE5534 in the second (RIAA) stage.
• rather high mm capacitance loading with 377pF total
• single 12au7 dual triode with no feedback, so high distortion and output impedance likely
• rectification inside the main box.

My unit arrived with a pleasant surprise. The tube was a PSvane red ECC82 rather than a generic China tube. As in the Stereoplay review that you can get a grainy pic of floating around, all opamps were NE5534.

The guts of mine:

MEASUREMENT APPROACH

I used my RME ADI-2 PRO FS R for measurements, driven by @pkane 's multitone software. It's excellent in this application, especially with the ability to apply an inverse RIAA curve to the generator to test RIAA accuracy. To limit time, I generated using the RME +4dBu range (1.23V) and +19dBu (6.9V) on the ADC (input) side. Yes, this means no CD level tests but you will soon see that is not too important.

LINE STAGE

At max volume, with the pot preceding the tube, and 1.23V in, this is the result below. Level out is -7.4dBFS wrt 6.9V = 2.94V, so ~7.6dB gain. Lots of distortion with this drive level, but very quiet with just the tube stage. Note, given the likely high output impedance, this will be higher into loads with a higher input impedance that the RME's 9kohm. I confirmed this by repeating a measurement into my NAD C3050LE, with an impedance of 28kohm (set to unity gain). Gain of the unit was around 3dB higher. With the two measurements, you can then solve the voltage divider simultaneous equations to estimate an output impedance of the TPR-2 of ~7150ohms.

Backing off the input a bit to 0.5V, still wide open improves things a bit. If you want to use a 2V source, you will need to keep the volume low to keep distortion reasonable.

A THD+N vs level sweep shows the gradual rise in THD:

For brevity, I wont show the very flat THD vs frequency curve, but here is the very good line input crosstalk:

Frequency response shows some drop-off at the lower end. Note this was 0.5dB better at 20 Hz into the higher impedance NAD:

This is probably enough on the line input, but here's CCIR at 0.5V in, volume set to unity gain. It again illustrates the flat distortion performance of something with no feedback:

PHONO STAGE
I wont show all 1kHz plots as they are affected by grounding, but at full volume, mm gain was 39.4dB, mc was 53.3dB. Which is strange, given the circuit show give 20dB higher for mc over mm. NOTE: input was 5mV, not 0.5mV!

RIAA response shows within +-0.5dB in the audio-band, with rolloff below and lift above (again title error was 5mV in):

OVERLOAD: reducing volume to "half" to be able to focus on the phono stage, overload for mm at 1kHz is ~69mV: Ignore +N given grounding issues.

For mc, it was about 12mV in, but a very soft clip. Again focus on THD rather than THD+N due to grounding. I hope to measure overload at other frequencies as well. Sorry for a different scale to the previous plot (dBV out rather than dBFS at the output).

Crosstalk with phono stage, again pretty good:

What about noise? After reading the LT1028 datasheet, it made for interesting reading about noise vs source impedance. Hence I measured noise with a cartridge attached, AT VM540ML for mm and AT OC9XEN for mc. Yes, I could have used resistors but attaching a cart was easier and perhaps more representative of use. I also obtained some (expensive) LT1028s and tried them (after soldering in DIL sockets):

• mm, NE5534, SNR 75dB, 78dBA
• mm, LT1028, SNR 65 dB, 67 dBA
• mc, NE5534, SNR 58dB, 64 dBA
• mc, LT1028, SNR 64 dB, 71 dBA

This seems expected for the LT1028, better noise for a low impedance mc but worse for the higher impedance mm.

Given the specs state a mm SNR of 68dB for mm and 70dB for mc, this is perhaps A weighted and for the LT1028 as was in the schematic. Close to what I measured.

CONCLUSIONS
It seems a competent design, within the constraints of the triode output! I have reduced the mm input capacitance to ~100pF and may try other opamp types (perhaps NE4434A-grade in an attempt to get a balanced noise performance between mm and mc. I will also investigate the mc gain and perhaps lift it up a bit by changing the first stage resistor switched in if the value is not as expected.

For driving a power amp the high output impedance may not be an issue, but may use is as a phono pre into the NAD C3050LE. Lucky it's input impedance is reasonable.

 

[Donutoftheheart]

really appreciate this!! Hard to find a ton of info on these preamps. I've recently upgraded my preamp and am now using the tpr3 as just a tube buffer stage. I was curious as to what gain/range I should fix the preamp to? And would it be unique to each tube? I'd like to avoid adding any excessive distortion as the audible sibilance as I increased the volume on my tpr3 is why I upgraded in the first place. Thanks!

 

[dougi]

really appreciate this!! Hard to find a ton of info on these preamps. I've recently upgraded my preamp and am now using the tpr3 as just a tube buffer stage. I was curious as to what gain/range I should fix the preamp to? And would it be unique to each tube? I'd like to avoid adding any excessive distortion as the audible sibilance as I increased the volume on my tpr3 is why I upgraded in the first place. Thanks!

I'm no tube expert, but have been playing with modelling the TPR-2 in LTspice to see if different tube types would make much of a difference. (conclusion is not much point given low plate voltage means can't really take advantage of higher gain 12XX7 series variants, you just get more distortion with a tiny bit of more gain). The TPR-3 has 6N3s, but circuit would be similar. So keep the volume somewhat below maximum unless you want THD in the 20s! The ideal point may change with exact tube, but given there is one per channel, you will need matched tubes anyway to avoid excessive channel imbalance.

 

[Listener]

I have one of these and it has the proper chip set, LT1028 first stage, NE5534 second stage. I am guessing that the model you have must have been manufactured during a chip shortage? In any case, I bought mine used and it arrived with a broken volume pot, which I replaced with a better quality pot. Also, as I live in the US I am running it off a step-up transformer, which should provide some isolation. I think that accounts for any differences between yours and mine. the bottom line is that I am very happy mine, it does its job and does it well, very quiet and present; listening conclusions, not testing conclusions. I have only tried it with a couple of mm carts Shure V15, Ortophon, I'll post again when I try out the mc side.

Listener

PS The circuit my audio equipment in on is a dedicated run, metal sheathing, copper ground wire, quality outlets... So I am not certain the the isolation from the step up transformer contributes anything.