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Tortuga TPB.V1 Tube Preamp Buffer Review

Francis Vaughan

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With the two transformers in there you could easily have made one of them provide, say, 200 V B+. Then use a CCS load and you get a very linear cathode follower.
That was exactly what I thought I was going to find when I first saw the PCB shot. It just got weird after that.
 

solderdude

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The guy is not after creating a high quality low distortion tube buffer.
I suspect he wanted as much harmonics.
Gain selection appears to just be 3 different cathode resistors.

This seems to be what's in the product:

inside.jpg


on the tube side there also seems to be a current source.

https://tortugaaudio.com/docs/tpb-v1-service-manual/
 

KSTR

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Tortuga show a basic circuit diagram for the single-ended version:
tube_buffer_simplified_schematic.jpg

Anode and (switchable) degeneration resistors not shown, but of course those are essential to make the first stage a (low-gain) inverter. In the balanced version I would guess they use a degenerated LTP with CCS bias and hopefully they got the polarity right (SE version inverts). Anyway, the tube will run with strong local feedback to reduce gain and total distortion, at the cost of introducing the full spray of upper harmonics.
 

KSTR

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I suspect he wanted as much harmonics.
I don't think so. He just didn't anticipate that someone would throw 4Vrms at it. ;-)
Up to 2Vrms I think the perfomance (THD -65dBc, 0.05%) is pretty acceptable with the stock tubes (at least with the left channel). I'd love to see a 1kHz spectrum at 1Vrms and several kOhm of input series resistance, mimicking a pot's worst case impedance (@amirm).
 

SIY

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redshift

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C’mon it can’t be that bad? The harmonics is clearly lower than the fundamental.

Sheez, these Puritan elitists... :mad:
 

levimax

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I don't think so. He just didn't anticipate that someone would throw 4Vrms at it. ;-)
Up to 2Vrms I think the perfomance (THD -65dBc, 0.05%) is pretty acceptable with the stock tubes (at least with the left channel). I'd love to see a 1kHz spectrum at 1Vrms and several kOhm of input series resistance, mimicking a pot's worst case impedance (@amirm).

At voltages up to 2Vrms, especially for the right channel, this thing is approaching transparency for listening to music. Seems like a lot of parts and expense to in essence "do nothing" (except make things worse that you can't hear). I guess if you put the wrong tubes in it at least it "does something" you can hear.
 

solderdude

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I'd love to see a 1kHz spectrum at 1Vrms and several kOhm of input series resistance, mimicking a pot's worst case impedance (@amirm).

Alas

The buffer is meant to be used after the LDR volume control. So when set to -20dB or so (what could be normal listening level) the THD of the volume control will be even higher than that of the buffer.
 
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welsh

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Starting in the 1950's when Hi-Fi became popular companies competed by making amplifiers as accurate/transparent as possible with the then available tube technology and many were quite successful i.e. distortion less than 1% and flat FR and they continued to improve through the 1960's with many tube amps achieving very good performance approaching even a modern definition of transparency. A good question I don't know the answer to is what was the first commercial tube amp that purposely degraded measured performance in order to "sound better"? Once this happened it seems like things have gone down hill and gotten crazy with low power and high distortion SET amps and "tube buffers" and the like becoming popular with the "subjectivist audiophile crowd". When I see products like this buffer it makes me sad that an interesting and useful technology that really moved consumer electronics forward is being reduced to expensive and poor performing "snake oil".
No sane audiophile in the 1960s would have used a SET amplifier. They were relegated to jukeboxes and radios. The adoption of this primitive technology by modern enthusiasts is simply perverse.
 

MakeMineVinyl

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No sane audiophile in the 1960s would have used a SET amplifier. They were relegated to jukeboxes and radios. The adoption of this primitive technology by modern enthusiasts is simply perverse.
No, in my case using a SET solves a specific problem. Sweeping generalities are generally not accurate. :facepalm:
 

Robin L

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No, in my case using a SET solves a specific problem. Sweeping generalities are generally not accurate. :facepalm:
I'd be curious to find out what problem got ironed out by a paleolithic design. I will confess to having heard that special SET magick a couple of times. It's all about the midrange.
 

MakeMineVinyl

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I'd be curious to find out what problem got ironed out by a paleolithic design. I will confess to having heard that special SET magick a couple of times. It's all about the midrange.
My speakers (horns - active crossovers) are 107db/w efficient from 500Hz up, and this is the driver which uses the SET. I use the SET to eliminate any potential for crossover distortion - the audibility of this form of distortion has been a problem in the past. These drivers, coupled to the large horns, are so sensitive that any low level grunge or noise upstream is audible. A PP class "A/B" tube amplifier or ultralinear amp would also be devoid of crossover distortion, but this topology would suppress even order harmonic distortion which then leaves the odd orders to irritate. The SET is heavily modified and uses feedback, so the harmonic distortion is typically <= .02% at most at the levels I listen at.

Also, since the SET is only capable of 2.5 watts, potential accidental damage to the diaphragms of the drivers is lessened.
 
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amirm

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Sorry, I have not seen any successful double blind test in audio.
I have post countless ones that I have passed (search for my alias and foobar). No way one can claim transparency for all people with such high levels of distortion.
 

redshift

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redshift

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here it is on a full track. he uses a more advanced version but in the T mode which is also available in the basic version

That sounded sweet in the mastering part of the video. I’ll bet this device can push the harmonics way over the fundamental when it is cranked to full tilt?

Distortion that doesn’t suck FTW.

:D
 
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