AIYIMA T20 Tube Pre-amplifier Review

[SSS]

View attachment 484438
Thank you, @SSS, perhaps you could show us the circuit diagram that you have in mind that utilises the 12AX7 in the FB Loop with the opamp (as the T20 overview shows) and measurements?

This is an excellent circuit (SRPP), as provided by @pma (thank you :=)), isn't it, which utilises the 12AU7 in the FB Loop with the opamp (as shown in the (above) circuit diagram and as the T20 overview shows), and graph/simulation of measurements show excellent measurements, don't they?

• Utilises both sides of 1 (dual Triode) Tube/Valve.
• Very low output impedance. Feeding Loads down to 600 ohm/Input Impedance, is no problem.
  • This implys that it will/should impedance match with any Device/Amp that it is Feeding, doesn't it?
• H2 but not too much (for those who enjoy, a little extra, H2), H3 below -120dB and no Higher H

View attachment 484439
View attachment 484440
Crosstalk
View attachment 484441

It was just to show there are other ways to use the tubes. Both examples are old known text book designs. Just replace the SRPP circuit with these. Perhaps the polarity for the negativ feedback need to be changed. Personally I do not like tube and op-amp combinations. So, no, I will not create a schematic. By the way, SRPP has also some challenges. One is the elevated cathode potential of the upper triode which needs also an elevated heater supply in order to stay in the specs for cathode - heater voltage difference. Can be omitted if low plate voltages are used but there may be the distortion would be higher.

 

[pma]

This is one of the working ways how to do it. Low output impedance, low distortion. I am not going to make it easy to our great wall friends, so I do NOT show the component values.

Changing the resistor values inside the FB loop and thus the feedback factor we get distortion numbers like with a SS preamp. Gain +23dB, 12AU7 tube (FYI, better than 12AX7 in this implementation), H2 distortion only, inaudible level. And masked by noise up to 3V. The only engineering question is why the tube, then. More heat, higher power consumption, limited tube life time.

 

[morillon]

Surely this should be in a different thread? This is supposed to be about the T20...

Sorry, I misunderstood your message...
Is there a chance to compare System Constant to other preamps?
;-)

 

[KL....]

Changing the resistor values inside the FB loop and thus the feedback factor we get distortion numbers like with a SS preamp. Gain +23dB, 12AU7 tube (FYI, better than 12AX7 in this implementation), H2 distortion only, inaudible level. And masked by noise up to 3V. The only engineering question is why the tube, then. More heat, higher power consumption, limited tube life time.

View attachment 484443

Thank you, @pma, this/above is not the SRPP graph/simulation (CL Gain +26dB) nor the T20, as follows, so (re Changing the resistor values inside the FB loop and thus the feedback factor we get distortion numbers like with a SS preamp. CL Gain +23dB (varyable?). The only engineering question is why the tube/12AU7/ECC82, then?), further Hints are welcome (linearity?), thank you?

To ask, what is the output V/I (Range, Min/Max for each?), for this implementation/circuit?

Also, perhaps, a question for the T20.... utilises SRPP or doesn't? If No, then @pma circuit diagram suggests that it can be (if desired), can't it, but 12AU7/ECC82 Tubes/Valves (better Headroom/Linearity, less N/D, Cooler Running?, better Tube Life?) are needed/required and resistor selection will be critical, won't it?

 

[Nzama]

Thank you, @pma, this/above is not your SRPP graph/simulation (CL Gain +26dB) nor the T20, as follows, so (re Changing the resistor values inside the FB loop and thus the feedback factor we get distortion numbers like with a SS preamp. CL Gain +23dB. The only engineering question is why the tube, then?), further Hints are welcome, thank you?

To ask, what is the output V/I?

Also, perhaps, a question for the T20.... utilises SRPP or doesn't?

View attachment 484487 View attachment 484489

How can it being balanced?!
You are already using the two sections of both tubes

 

[KL....]

How can it being balanced?!
You are already using the two sections of both tubes

Thank you, do you mean Matched Tubes? If Yes then 2 channel/stereo and matching/balance is corrected/controlled via CL FB for each opmap + tube/channel. Note that @pma has advised matching is not critical and further note that @pma can easyly better/best advise you, especially as this is @pma's Circuit diagram, so hopefully he will do so (if and should he feel inclined to do so or (perhaps) he might like you to keep striveing/guessing/learning) :=)

Do you mean Balanced input? If yes then via the NJW1195.

 

[Music+Coffee]

With stock tubes, mids seem a little recessed. Switching to old Sylvania (long grays) brings the mids a bit forward and also sounds more airy. Not sure if what I'm hearing is due to more distortion and the airiness due to microphonics but I kinda like it with the Sylvania tubes. The rest of the system are budget/pro-audio stuff: Presonus Studio 8s, Rega Planar 1 turntable and Schiit Audio Skoll. DAC is an AUNE X8 (considering on replacing it with Schiit Mimir).

 

[Elvom]

I didn’t order the trigger cable for the T20 to the A20! Can someone confirm if the trigger cable is TS or TRS please?

Just got mine, no cable trigger on the t20, im my case, the a20 did had one trigger cable included

 

[Music+Coffee]

Just got mine, no cable trigger on the t20, im my case, the a20 did had one trigger cable included

No trigger cable on my T20 as well.

 

[KL....]

Thank you, the following is for your consideration and comment....

• Note that the question still is, for the T20.... does it utilise SRPP or doesn't it? If yes then perhaps, the PSVANE Horizon 12AX7-AT could/should be considered (perhaps as a purchase option, maybe?) but it is not the 12AU7, is it?
• @AIYIMA can advise, can't they?

SRPP…. Series Regulated Push Pull

• An op-amp and SRPP circuit using a 12AU7 tube (or other tubes like the 6SN7/6DJ8/E88CC) is a hybrid design that can create a preamplifier or amplifier by combining the characteristics of both technologies. One approach (Opamp +Tube) is to use the SRPP (Series-Regulated Push-Pull) circuit to handle the signal amplification, while the op-amp is used to improve performance metrics such as noise reduction, power supply rejection (PSRR), and/or to provide a feedback loop/FB for precise control.

Op-amp and SRPP 12AU7 hybrid circuit

• SRPP stage: Typically, an SRPP circuit uses two the triodes/both sides of a 12AU7 tube (or a 6SN7/6DJ8/E88CC/etc as shown) to form a gain stage. The first triode acts as a CCS (Constant Current Source), and the second triode acts as an Active load, allowing for high voltage gain and low output impedance. The 12AU7 tube is a common choice for this configuration because it contains Two/Dual triodes per tube, making it easy to build a stereo SRPP circuit.
• Op-amp integration: The op-amp can be used to improve the SRPP's performance. For example, an Opamp can be used in a feedback loop to reduce output noise or improve the PSRR (Power Supply Rejection Ratio) by adding an anti-phase ripple-induced current flow that nulls the PS noise at the output (O/P).
• Complete circuit: In a practical circuit, the Opamp + SRPP can be combined to form a Preamplifyer. The SRPP stage can handle the bulk of the Amplification, with an Opamp providing a controlled output and/or feedback loop to fine-tune performance. This hybrid approach leverages the Tubes characteristics for a pleasing sound while utiliseing the Opamp to address some of the inherent limitations of Tubes, such as Noise and PS imperfections.

Design considerations

• Tube choice: The 12AU7 is a common choice due to its Dual/Triode design, but other Tubes like the 6SN7/6DJ8/E88CC/etc can also be utilised. For the output stage, a 12BH7 might offer more open and dynamic sound than a 12AU7, according to Scribd.
• Power supply: A high-voltage PS is required for the Tube, and a separate regulated DC supply may be used for the Heaters to reduce Hum and Noise.
• Component selection: Resistor and Capacitor values will need to be carefully selected based on the specific Tube and desired performance metrics. For example, the plate resistors (RL) are typically chosen between 150kΩ to 220kΩ, and the Cathode resistors (RK) are 1.5kΩ for a 12AU7, notes ttradio.net.
• Performance tuning: Measurements like CCIF IMD and output Noise can be used to evaluate and optimize the Circuits performance, as shown in the Audio Science Review (ASR) Forum review of the GlassWare SRPP+ kit.

 

[KL....]

Thank you, no further comment thus far, so....

• SRPP Decoded.... This circuit has many names: SRPP, SEPP, Totem Pole, Mu Follower, Mu amplifier, Cascoded Cathode Follower, and its original name, the Series-Balanced amplifier. Just what SRPP means is uncertain; maybe it stands for Series Regulated Push-Pull amplifier or Single-Ended Reflexive Push-Pull amplifier. This circuit promises the advantages of both the Grounded Cathode amplifier and the Cathode Follower.... high gain and low output impedance. Yet, in spite of its popularity, few realize that it is not a single-ended circuit. In fact, most tube fanciers are shocked when told that this circuit is fundamentally a push-pull amplifier and not that radically different than the output stage of the a push-pull power amplifyer.
• Hint.... As indicated/shown, the Amplifyed SRPP (Push-Pull) Sine Wave output (about 5Vpp, Class A), as Simulated by @pma. Input Sine Wave value not indicated but as the 12AU7/ECC82 Amplification Factor (μ/mu) is 20 then Input is (about) 0.25V, isn't it? @pma, would this be the Vmax for your Simulated SRPP output?
  • Further Hint.... it looks very good, indeed, doesn't it?
• 
• Further, @pma indicates for reference, that the Crosstalk, measured with the tube SRPP (12AU7/ECC82) board (no op-amps) as follows, is good enough but depends on the PCB design and plate supply design. The channels are independent, one tube per channel. Also, the Dynamic Range is 117dB A-weighted and that this is really "not bad".
• 

Hint.... The LOV (Biasing/Load resistor, which determines the output impedance and the overall current flow) is replaced by the Opamp and GFB. For discussion/comment, review and look at the previous posts....
Hint.... Can the Opamp be regarded as a Buffer for this arrangement (Further Hint... usually Unity Gain, but as the SRPP handles the bulk of the Gain/Amplification, with the Opamp providing a controlled output and feedback loop to fine-tune performance, then reasonable?). For discussion/comment, review and look at the previous posts....
Hint.... V/I/Resistor/Cap values are Critical. For discussion/comment, review and look at the previous posts....
Hint.... Ask @pma any question/s, especially as this is his Simulated Circuit/Measurement, as he will know and be better/best to ask, won't he :=)
This

is now

Note that the question still is, for the T20.... does it utilise SRPP or doesn't it? It certainly has a 12AX7 & Opamp per channel, doesn't it?

• Can you place a 12AU7/ECC82 in a T20.... Definitely not, No, not without (careful) consideration for the circuit (which we do not know? @AIYIMA can advise, can't they?), as indicated by previous posts, the Tubes V/I values are critical and/as such the resistor/cap values are critical, aren't they?
• The 12AU7/ECC82 Tube
• The 12AX7/ECC83 Tube

 

[Cerwin]

Does anyone know if the RCA and XLR outputs can be used simultaneously? In my actual preamp I use the XLR output to feed my power amplifier and the RCA outputs to feed a subwoofer and I apply the frecuency filter in my sub.

Also I miss the recommended input signal voltaje for RCA and XLR and the voltage output to set the optimus gain in my power amp.

Thanks.

 

[KL....]

Does anyone know if the RCA and XLR outputs can be used simultaneously? In my actual preamp I use the XLR output to feed my power amplifier and the RCA outputs to feed a subwoofer and I apply the frecuency filter in my sub.

Also I miss the recommended input signal voltaje for RCA and XLR and the voltage output to set the optimus gain in my power amp.

Thanks.

Thank you, @Cerwin, the @AIYIMA overview suggests/indicates that you can because the XLR/RCA are driven by separate LM4562 outputs. Note that the LM4562 XLR L/R circuit outputs are feeding one LM4562 RCA circuit L/R inputs and another Device/s (L and/or R) XLR input, don't they? Note, with reference to the T20 output/feeding to other/another Devices Input Impedance/Load, that 2 XLR and 1 RCA (active Sub) connections will be better than 2 RCA and 1 XLR (active Sub) connections, why?, because 1 XLR connection would imbalance the feed/ing to the LM4562/RCA circuit, won't it, although 2 XLR (same Device/s) and 2 RCA (same Device/s) connections will be balanced/fine, won't it? @AIYIMA can advise, can't they?

Also I miss the recommended input signal voltaje for RCA and XLR and the voltage output to set the optimus gain in my power amp.

As such, the (another/XLR) Devices Input Impedance (Loading) will need to be considered when RCA and XLR Devices are both connected/active, won't it, but the RCA should be independent/fine, won't it? @Cerwin, what is the Input Impedance of your Amplifyer and are you using XLR or RCA (note below/as follows, RCA is possablely better)? Note that we do not know the T20 min/max V/I/Output Impedance values do we (or do we?), so if you know for XLR/RCA, let us know? For the T20 XLR/RCA Inputs, let us know what you have deduced/considered, thus far :=)

The consideration for the XLR Devices Input Impedance implys/means that the RCA is possablely higher performing than the XLR?.... and Yes (just looked), the measurements indicate Yes, don't they, which is not a surprise, is it? Further, re Measurement, was the T20 measured with an RCA Load and XLR Load both connected which is not indicated, so and as such, we do not know, do we?

Further, the T20, noteing the above consideration/s, will allow you to utilise.... 2 (active, without Y adapter) subs and 1 stereo amp, or 2 (active, without Y adapter) subs and 2 mono amps, or 2 stereo amps, or 4 mono amps, if needed/required....

 

[KL....]

Thank you, @Cerwin, updated reply, for you :=)

 

[AIYIMA]

Thank you, the following is for your consideration and comment....

• Note that the question still is, for the T20.... does it utilise SRPP or doesn't it? If yes then perhaps, the PSVANE Horizon 12AX7-AT could/should be considered (perhaps as a purchase option, maybe?) but it is not the 12AU7, is it?
• @AIYIMA can advise, can't they?

SRPP…. Series Regulated Push Pull

• An op-amp and SRPP circuit using a 12AU7 tube (or other tubes like the 6SN7/6DJ8/E88CC) is a hybrid design that can create a preamplifier or amplifier by combining the characteristics of both technologies. One approach (Opamp +Tube) is to use the SRPP (Series-Regulated Push-Pull) circuit to handle the signal amplification, while the op-amp is used to improve performance metrics such as noise reduction, power supply rejection (PSRR), and/or to provide a feedback loop/FB for precise control.

Op-amp and SRPP 12AU7 hybrid circuit

• SRPP stage: Typically, an SRPP circuit uses two the triodes/both sides of a 12AU7 tube (or a 6SN7/6DJ8/E88CC/etc as shown) to form a gain stage. The first triode acts as a CCS (Constant Current Source), and the second triode acts as an Active load, allowing for high voltage gain and low output impedance. The 12AU7 tube is a common choice for this configuration because it contains Two/Dual triodes per tube, making it easy to build a stereo SRPP circuit.
• Op-amp integration: The op-amp can be used to improve the SRPP's performance. For example, an Opamp can be used in a feedback loop to reduce output noise or improve the PSRR (Power Supply Rejection Ratio) by adding an anti-phase ripple-induced current flow that nulls the PS noise at the output (O/P).
• Complete circuit: In a practical circuit, the Opamp + SRPP can be combined to form a Preamplifyer. The SRPP stage can handle the bulk of the Amplification, with an Opamp providing a controlled output and/or feedback loop to fine-tune performance. This hybrid approach leverages the Tubes characteristics for a pleasing sound while utiliseing the Opamp to address some of the inherent limitations of Tubes, such as Noise and PS imperfections.

Design considerations

• Tube choice: The 12AU7 is a common choice due to its Dual/Triode design, but other Tubes like the 6SN7/6DJ8/E88CC/etc can also be utilised. For the output stage, a 12BH7 might offer more open and dynamic sound than a 12AU7, according to Scribd.
• Power supply: A high-voltage PS is required for the Tube, and a separate regulated DC supply may be used for the Heaters to reduce Hum and Noise.
• Component selection: Resistor and Capacitor values will need to be carefully selected based on the specific Tube and desired performance metrics. For example, the plate resistors (RL) are typically chosen between 150kΩ to 220kΩ, and the Cathode resistors (RK) are 1.5kΩ for a 12AU7, notes ttradio.net.
• Performance tuning: Measurements like CCIF IMD and output Noise can be used to evaluate and optimize the Circuits performance, as shown in the Audio Science Review (ASR) Forum review of the GlassWare SRPP+ kit.

View attachment 484799 View attachment 484670

Hi, thank you for your insightful technical question.

The T20 does not use an SRPP circuit, mainly due to its fully balanced design.
A fully balanced architecture requires handling four signal paths, while an SRPP circuit consumes two triodes per path.
That means a total of four 12AX7 tubes (eight triodes), which would significantly increase both cost and thermal design pressure.

During development, we had to balance circuit complexity, overall performance, reliability, and price.
The final design represents what we believe to be the best overall sonic performance achievable at this price point.

We hope this explanation helps you better understand our design philosophy and technical considerations.

 

[AIYIMA]

Thank you, @Cerwin, the @AIYIMA overview suggests/indicates that you can because the XLR/RCA are driven by separate LM4562 outputs. Note that the LM4562 XLR L/R circuit outputs are feeding one LM4562 RCA circuit L/R inputs and another Device/s (L and/or R) XLR input, don't they? Note, with reference to the T20 output/feeding to other/another Devices Input Impedance/Load, that 2 XLR and 1 RCA (active Sub) connections will be better than 2 RCA and 1 XLR (active Sub) connections, why?, because 1 XLR connection would imbalance the feed/ing to the LM4562/RCA circuit, won't it, although 2 XLR (same Device/s) and 2 RCA (same Device/s) connections will be balanced/fine, won't it? @AIYIMA can advise, can't they?

View attachment 485084


As such, the (another/XLR) Devices Input Impedance (Loading) will need to be considered when RCA and XLR Devices are both connected/active, won't it, but the RCA should be independent/fine, won't it? @Cerwin, what is the Input Impedance of your Amplifyer and are you using XLR or RCA (note below/as follows, RCA is possablely better)? Note that we do not know the T20 min/max V/I/Output Impedance values do we (or do we?), so if you know for XLR/RCA, let us know? For the T20 XLR/RCA Inputs, let us know what you have deduced/considered, thus far :=)

The consideration for the XLR Devices Input Impedance implys/means that the RCA is possablely higher performing than the XLR?.... and Yes (just looked), the measurements indicate Yes, don't they, which is not a surprise, is it? Further, re Measurement, was the T20 measured with an RCA Load and XLR Load both connected which is not indicated, so and as such, we do not know, do we?

View attachment 485085 View attachment 485086

Further, the T20, noteing the above consideration/s, will allow you to utilise.... 2 (active, without Y adapter) subs and 1 stereo amp, or 2 (active, without Y adapter) subs and 2 mono amps, or 2 stereo amps, or 4 mono amps, if needed/required....

Choosing 2 XLR connections (left and right channels) plus RCA for the subwoofer is the right approach.
XLR cables offer excellent interference resistance, which ensures better balance and stability for the left and right channels, delivering cleaner and more precise sound.

 

[toilagamer852]

A question from a casual audiophile. How much (or in what way) does the T20 benefit my A07 Max amp, or do I need matching amp like A20? Eventually I'll change to A20 anyway but not before I purchase a subwoofer.

 

[SMen]

A question from a casual audiophile. How much (or in what way) does the T20 benefit my A07 Max amp, or do I need matching amp like A20? Eventually I'll change to A20 anyway but not before I purchase a subwoofer.

I think the order is probably wrong for benefiting except marginally / if at all - A20 looks more of an upgrade and more suitable for a subwoofer to buy first. I am using a T20 with a 3e audio A7 - and loving it ... but I (to my ears anyway) tweaked it up with a better power supply and Tung-sol valves.

I think you might not experience the T20 benefits by going to this first is what I am saying.

 

[Uncola]

Guys, I set the t20 preamp up today but my trigger outs don't seem to be working. I tried 3.5mm trs cables I got from benchmark which didn't work so I pulled the 3.5mm mono cables from my denon receiver trigger outs, which I verified worked with the actual amplifiers I'm trying to use with the t20.. and still not working did anyone figure out how to use them?

 

[AIYIMA]

Guys, I set the t20 preamp up today but my trigger outs don't seem to be working. I tried 3.5mm trs cables I got from benchmark which didn't work so I pulled the 3.5mm mono cables from my denon receiver trigger outs, which I verified worked with the actual amplifiers I'm trying to use with the t20.. and still not working did anyone figure out how to use them?

You need to use a 3.5mm stereo cable.