[rafael_triode]

Hi fellows! anyone tried other preamps with the Thoress 300B or 845 monos other than the Thoress preamps? I read in a review that they need a preamp providing a lot of output V to be compatible with the Thoress.

[FullRangeMan]

Hi fellows! anyone tried other preamps with the Thoress 300B or 845 monos other than the Thoress preamps? I read in a review that they need a preamp providing a lot of output V to be compatible with the Thoress.

I will move this topic to the Tubeophile Circle>
https://www.audiocircle.com/index.php?board=144.0

You would check how much is the power amps input sensitivity to find out what are the values you have on these amps. It may be need just a little of gain in the signal on the pre-amp IMO.

[rafael_triode]

The 300B Thoress input sensitivity is 3.25 V and input impedance is 100K ohm. According to the manual, the amp needs a preamp with more than 2.71 V total.
I tried with my Allnic L3000 preamp (output voltage: 15V, voltage gain:+20dB, output impedance: 150 ohm) and the sound has no dynamics at all and has very low volume even if I turn the preamp volume knob higher and higher... I don’t understand why it doesn’t work because the Allnic specifications should match the Thoress specifications.
I searched for the Thoress line stage specifications, but there is no information about output in the manual and anywhere. So I can’t know what the manufacturer made with his preamp in order to to compare with other similar preamps. 
In a Stereophile review, Art Dudley connected a Shindo Masseto in the Thoress 300B. According to Shindo website (http://www.shindo-laboratory.co.jp/English/Masseto.html), the Masseto has Output/ Impedance：5V/600Ω.

[FullRangeMan]

Looks like your speakers are low sensitivity.

[rafael_triode]

Good possible avenue, but it’s not even that! The speakers are horn Avantgarde Duo, impedance: 18Ω. Sensitivity: 107dB.
It worked very well even with my previous monos with 45 valve and only 2 watts with my Allnic L3000 preamp. Now i have Shindo Corton Charlemagne EL34 monos that work very well also, but they give 25 watts.

[rafael_triode]

Looks like your speakers are low sensitivity.

Part of the answer is maybe in this Thoress monos review (https://www.tnt-audio.com/ampli/thoress_300b_e.html): A feature of this design is a relatively low overall gain, this means you need a source that has a high output, i.e. it delivers lots of volts. In my setup it worked fine but you should pick your pre-amplifier with care to ensure compatibility.

I thought the Allnic 15V should be enough in theory, but in practice….

[FullRangeMan]

By what you said the prob is in that specific amp only, the speakers seems fine.

[FullRangeMan]

Part of the answer is maybe in this Thoress monos review (https://www.tnt-audio.com/ampli/thoress_300b_e.html): A feature of this design is a relatively low overall gain, this means you need a source that has a high output, i.e. it delivers lots of volts. In my setup it worked fine but you should pick your pre-amplifier with care to ensure compatibility.

I thought the Allnic 15V should be enough in theory, but in practice….

The gain is in the input circuit, usually the input gain is fixed and cannot be adjusted. The regular volume control all preamps have refers to the output level.

[opnly bafld]

Something is amiss.
What is your source? Are all tubes known to be good?
The Allnic L3000 has 20 dBs of gain which should be plenty especially with your high sensitivity speakers.
The TNT reviewer used a passive unity gain (volts in = volts out) preamp with a 5.85v source and was able to use 96 dB speakers.

[FullRangeMan]

A output tube senile is a good guess.

[rafael_triode]

Something is amiss.
What is your source? Are all tubes known to be good?
The Allnic L3000 has 20 dBs of gain which should be plenty especially with your high sensitivity speakers.
The TNT reviewer used a passive unity gain (volts in = volts out) preamp with a 5.85v source and was able to use 96 dB speakers.

The source is a Rega Isis CD player.
The Thoress output tubes are new condition 300B-XLS Emission Labs (XLS = higher power). The driver tubes are the original CV2382. All tubes seem works flawlessly. I don’t have yet extra CV2382 to give a try. 
As you noted in the reviews, TNT and Art Dudley used preamps with much less Volts than my Allnic 15V! The gain are similar.

[JakeJ]

Any chance you have access to another higher gain preamp?  For example, I have an Audio Research LS25 MkII (uses two 6H30 tubes) that features gain selections of 0dB, +6dB, and +12dB.  I've had other preamps over the decades that had gain switches and one thing I have learned is  NO switching until the preamp is muted.  No mute button?, turn the amp(s) off first.  Trust me on this.

[opnly bafld]

Since you can run either XLR or RCA from the Rega to the Allnic have you tried both ways?