[matheusrhi]

Hi, the Muse TU-20 could drive my AKG K240 Sextett 600 ohms?
 
Thank you!

 

[MDR30]

Quote:

Hi, the Muse TU-20 could drive my AKG K240 Sextett 600 ohms?
 
Thank you!

 
I don't have the Sextett, but I've tried the K140 (also 600 ohms and probably the same driver, the forerunner headphone of the K240) and the K340 and the Muse drives them both without problems. With a Sansa Clip as source it plays loud enough with the volume control at 11-12. Quite impressive, really.
 

 

 

[matheusrhi]

Thank you. I was in doubt because I did not find enough information on the maximum output impedance. I want to buy a low cost amplifier and found interesting the TU-20.
I also drew attention Bravo V1 and V2. Could tell me which would be better worth between TU-20, Bravo V1 or V2 (or any other)?

 

[MDR30]

Quote:

Thank you. I was in doubt because I did not find enough information on the maximum output impedance. I want to buy a low cost amplifier and found interesting the TU-20.
I also drew attention Bravo V1 and V2. Could tell me which would be better worth between TU-20, Bravo V1 or V2 (or any other)?

 
I don't have any experience of the other amps you mention, but the Indeed G3 may be something to consider. Visit rock grotto for more information.

 

[bhazard]

Does this really put out 3W per channel at 32ohm like it is listed? I like that it can use different tube families after desoldering.

 

[vic2vic]

  There was a distinct difference when lowering the bias from 7,2 to 6,3 V. Maybe some nice distorsion that gave the sound an edge disappeared. Riding that voltage close to the border may be the trick. I don't have an oscilloscope, but someone who has could probably find the right setting.
 
In the meantime I got myself some op amps (cheap from Hong Kong) and a tube to try out next week.
 

 

 
Hi MDR30 (and others), sorry for resuming an old thread, but it seems this is the only one in HF related to the Muse TU-20.
I've one of this tube amps on its way and I'd like to order a couple of opamps to roll with it.
Any impression on the 3 ones that you have listed above ?
 
One more noob question on the substitute opamp: to replace the existing NE5532, should I go for the adapters which have only 1 one opamp mounted on top  (http://www.ebay.com/itm/1-Pieces-AD797AR-SOIC-to-DIP-Op-amp-replace-NE5534-/120757516362?pt=LH_DefaultDomain_0&hash=item1c1db5784a) or the ones with 2 opamps solder on top and bottom faces (http://www.ebay.com/itm/120757521995?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649) ?
Thanks for your help.

 

[MDR30]

Hi MDR30 (and others), sorry for resuming an old thread, but it seems this is the only one in HF related to the Muse TU-20.
I've one of this tube amps on its way and I'd like to order a couple of opamps to roll with it.
Any impression on the 3 ones that you have listed above ?

One more noob question on the substitute opamp: to replace the existing NE5532, should I go for the adapters which have only 1 one opamp mounted on top  (http://www.ebay.com/itm/1-Pieces-AD797AR-SOIC-to-DIP-Op-amp-replace-NE5534-/120757516362?pt=LH_DefaultDomain_0&hash=item1c1db5784a) or the ones with 2 opamps solder on top and bottom faces (http://www.ebay.com/itm/120757521995?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649) ?
Thanks for your help.

Go for the ones in my pics. AD825 and AD797 are one channel op amps (if I remember correctly) so you need double assemblies. I've moved on from this amp so can't really remember the difference of the op amps but I notice I kept the AD825 in the amp.

However, I think a change to the ECC86 tube would make a bigger difference as this was designed for a lower plate voltage, for use in car radios in the late 1950s, and would work better in this circuit. You can probably find it at a good price on ebay, but you need to adjust the bias voltage as described in this thread (which really goes for the op amps as well).

 

[vic2vic]

Go for the ones in my pics. AD825 and AD797 are one channel op amps (if I remember correctly) so you need double assemblies. I've moved on from this amp so can't really remember the difference of the op amps but I notice I kept the AD825 in the amp.

However, I think a change to the ECC86 tube would make a bigger difference as this was designed for a lower plate voltage, for use in car radios in the late 1950s, and would work better in this circuit. You can probably find it at a good price on ebay, but you need to adjust the bias voltage as described in this thread (which really goes for the op amps as well).

Thanks for the help.
As main tube amp, I already have a Little Dot MK II, with some mods to run 6922/6dj8 tubes (and 12AU7 /12AX7 too). So on tube side I have already some spare ones to be rolled in the Muse.
The TU-20 is actually a "downgrade", in the sense that I needed a cheaper amp to be able to start practicing with opamp rolling (which I never did before) and some easy board mods (e.g. capacitors upgrades). I was able to get it for quite a cheap price, so I preferred to go for this one instead of the Indeed G2 / G3 (even if these 2 have a much wider story for modding in rockgrotto forum). And if I break it, I still have a nice metal case
 
By the way, the amp is still on its way, so it will take a bit before practicing with these mods.

 

[MDR30]

  Thanks for the help.
As main tube amp, I already have a Little Dot MK II, with some mods to run 6922/6dj8 tubes (and 12AU7 /12AX7 too). So on tube side I have already some spare ones to be rolled in the Muse.
The TU-20 is actually a "downgrade", in the sense that I needed a cheaper amp to be able to start practicing with opamp rolling (which I never did before) and some easy board mods (e.g. capacitors upgrades). I was able to get it for quite a cheap price, so I preferred to go for this one instead of the Indeed G2 / G3 (even if these 2 have a much wider story for modding in rockgrotto forum). And if I break it, I still have a nice metal case
 
By the way, the amp is still on its way, so it will take a bit before practicing with these mods.

 
 
Good luck! And trust your ears, in the long run they're your only reference.

 

[MDR30]

  I actually have that little amp with me on the road. I carry a Corda 2Stepdance with me and it is my favorite potable amp. The HD800 sounds good with everything. This little tube amp was just a toy to play with. The AD797AR and my APEX tube sounded best, with a clean bias setting. I find the impedance match is better with the line in ports on the back. I have placed poly bi-pass caps across every capacitor to reduce noise and replaced the power supply with a much better unit. I think I got the best I can get it to sound, and have not really played with it much. There really isn't enough power to drive the tube to it's potential other then over modulation.

When you say "isn't enough power", are you talking about plate voltage or the power supply? I tried the ECC86 tube, designed for car radios and 6-12 V (maxiumum 30 V) plate voltage, and that seems to work very well. What 's the optimum setting for the op amp - 6-7 V or higher?

 

[vic2vic]

   
Also soldered a ground lead. It locks between back plate and box.
 

 
 

 
Just got my TU-20 on Christmas' Eve. After a short audition out of a Fiio X3 it sounds ok, but just after third volume notch it gets quite noisy (humming).
As possible mitigations I saw the suggestions in previous posts:
- grounding (as in the picture above). Question 1: how is the is the ground lead connected to the case ? Can I just screw it to the closest case screw (e.g. the one next to the black "foot" in the picture above) ?
- improved wall-wart with a more powerful one. Question 2: the power supply included with my TU-20 is a 12V - 3A, while in previous posts there are suggestions to use a 12V -2.5A one. Isn't the one already included with my amp powerful enough ? If not (as real power could be different from the one on the label), how can I measure its actual power ? I just got a multimeter, but not sure how to use it for this check on a wall-wart.
 
Thanks
 
Sorry for my noob questions, but I'm just learning "on the field" how to tweak this amp

 

[sergeich]

Recently bought this amp. 
"out of the box" was heard only noise. 
Made ground and lowered the voltage to 6.3V, became perfectly sound. 
Use Creative Elite Pro, headphone Audio-Technica ATH-A900X. 
Warming up over 100 hours.
Wanted to know whether to put ECC82, or leave it as is?

 

[MDR30]

   
Just got my TU-20 on Christmas' Eve. After a short audition out of a Fiio X3 it sounds ok, but just after third volume notch it gets quite noisy (humming).
As possible mitigations I saw the suggestions in previous posts:
- grounding (as in the picture above). Question 1: how is the is the ground lead connected to the case ? Can I just screw it to the closest case screw (e.g. the one next to the black "foot" in the picture above) ?
- improved wall-wart with a more powerful one. Question 2: the power supply included with my TU-20 is a 12V - 3A, while in previous posts there are suggestions to use a 12V -2.5A one. Isn't the one already included with my amp powerful enough ? If not (as real power could be different from the one on the label), how can I measure its actual power ? I just got a multimeter, but not sure how to use it for this check on a wall-wart.
 
Thanks
 
Sorry for my noob questions, but I'm just learning "on the field" how to tweak this amp

 
You've probably solved this problem already, but I'll answer anyway for the community.
 
1) I soldered one end of the lead to ground on the board. The other end was placed between box and back lid before the latter was screwed in place.
 
2) I found that mains hum differed between wall warts (and to a lesser extent the signal source - portable player/mains connected player). The one supplied with my amp (230V mains) had an inherent hum. I tried a few different wall warts and found a couple with no mains hum. You just have to try for yourself. You can also try turning the mains plug 180 degrees in the wall outlet, if that is possible in your country. Sometimes helps.
 
Haven't found a better amp for the price (both power and perfomance) and chip as well as tube rolling is worthwhile and fun. More of that later.

 

[MDR30]

Had some time to spend and decided to dig this little unit out of the cupboard for a more elaborate evaluation.
 
Did some listening with different op amps and tubes, and noticed that the tube made the most noticable difference.
 

 
Of the five chips I had available I thought the supplied Texas Instruments NE5532P and a Burr Brown 2604AP slightly less open and defined than the adapter back to backs: ADR825, AD797 and 627AU. Could'nt hear much difference between these three but the AD797 double pack ended up in the Muse.
 
Tube rolling was quite interesting. As shown earlier in this thread a small mod makes it possible to switch between 6V heater tubes (ECC88, ECC86) and 12 V tubes (12AU7). Here's the jumper in place for a 12AU7 tube (ECC82).
 

 
I also decided to listen while I did the circuit's voltage adjustment, described earlier in the thread. I just went for the sweet spot with cleanest and strongest output (distorsion and imbalance are obvious). I did this by ear as I have no oscilloscope, at normal to somewhat louder listening levels with 120 ohms AKG and 150 ohms Sennheiser headphones. I the did the final adjustment with a volt meter to get the same left/right voltage from the op amp's output.
 
Changing op amps hardly changed the output voltage, but switching tube certainly did. Not only type (I tried the three mentioned above) but brands and equivalents.
 
As I understand, in this kind of circuit op amp and tube amplifier stages must be balanced. Voltage adjustment also slightly affects heater and plate voltage as they are part of the same circuit.
 
The amp was supplied with a Russian made Electro Harmonix 6922 (ECC88) which does its job well, but I wanted to see what other types could do.
 
The ECC86 is a dark horse in this company. It was designed as an input tube for car radios in the late 1950s, driving output transistors. Much the same configuration as the TU20! Its main goal was to work properly with 6, 12 or 18V car batteries which seems perfect for the TU20. Voltages much different from the ECC88 wich often runs at 90V, or the ECC82 at 150V. But these tubes apparently work for use in low voltage circuits like these anyway.
 
I want make it clear that the voltage adjustments suggested below are what I preferred in these specific settings, with these specific tubes. They are just guidelines to make it easier to find the best voltage setting of your own favourite tube. They are all different, even of the same type/brand.
 

 
 
ECC86 (6GM8, 6N27P) 6V heater
Preferred adjustment voltage: 4V
Plate voltage at adjustment above: 4V
 
Sounds surprisingly good: warm, neutral and relaxed, even though the Muse circuit values probably were not chosen for this tube (but it is very close to the ECC88 except for its 30V maximum plate voltage rating). I hade three different tubes: two Philips Miniwatt and a Haltron branded one which sounded quite similar. Can probably be found quite cheap.
 
 
ECC88 (6DJ8, 6N23P) 6V heater
Preferred adjustment voltage: 6V
Plate voltage at adjustment above: 5,9 V
 
Works very well, only had a Mullard ECC88 (NOS) which sounded excellent - fine balance between clarity, warmth and space. Top competer.
 
 
E88CC (6922) 6V heater
Preferred adjustment voltage: 7-7,5V
Plate voltage at adjustment above: 7-7,5V
 
My tubes of this type were quite different. The special durable version E88CC (6922) needs slightly higher voltage adjustments than ECC88 according to my findings. Russian made Electro Harmonix 6922 was robust and powerful, but sections were not very well matched - very difficult to adjust. Philips 6922 JAN seemed a bit strident in this configuration. Telefunken and Mullard gold pins E88CC were quite different from each other: first one detailed and transparent, the latter warm and lush. Take your pick. Found this tube type difficult to adjust properly. Top choice would probably be the Telefunken gold. NB: trying a lower voltage, say 6.3 volts if adjustment allows, may result in a softer sound.
 
 
ECC82 (12AU7) 12V heater - MOD REQUIRED AT YOUR OWN RISK, SEE POST #30!
Preferred adjustment voltage: 6,3V
Plate voltage at adjustment above: 10,3V
 
This tube can only be used if you open the amp and desolder a jumper (see post #30). Classic preamp tube. I only had a couple different ones, must stock up. Clarity, precison and a fine sound stage made me prefer the ECC82 this afternoon so it stays in the amp (East German RTF). Maybe the higher plate voltage did the trick... I'll be able to make better conclusions when I've tested a few more ECC82s and have let them burn in properly in the amp.
 
 
CONCLUSION
 
This is a very capable amp at its price. Selecting  a good opamp and a suitable tube, adding some high quality caps - see post #21 - and that's as good as this amp's gonna get. Hard  to beat when it comes to value for money.
 
On the other hand, for a little more you get an Indeed G3 with input tube and output MOSFETs. This class A hybrid takes a lot of different loads and can, in my opinion, compete with more expensive amps (not that I've heard them all).
 
But it sure is fun getting the best out of these tiny and affordable toys.
 

 

[Shensmobile]

Sorry to revive this but I decided to pick one of these up since everyone seems to be enjoying their TU20 so much.  While it sounds great, after changing the ACDC wall wart to a better quality one, the gain was way too high.  I contacted the eBay seller to ask which resistors I could change to lower the gain.  He said R6 and R8, and confirmed that lowering the resistors should decrease the gain.
 
The original ones were 1k ohm, and I tried 360 ohm and then 100 ohm.  Each time, gain seemed to increase.  Not trusting the eBay seller, I took it apart and probed the board.  The op-amps seem to output like this:
 
-OpAmp output is connected to OpAmp (-) input.
-OpAmp output/OpAmp (-) input connects to the 470 uF cap (coupling cap)
-Other side of 470 uF cap is connected to 47 ohm resistor
-Other side of 47 ohm resistor is connected to headphone jack AND a 1k ohm resistor (this is the ones they were asking me to DECREASE)
-Other side of 1k ohm resistor is connected to ground.
 
So it appears that the op-amps are not actually doing any amplification as this is Unity Gain configuration.  The resistors are essentially attentuators.  This is a disappointment personally.
 
Does anyone know how much op amps colour the sound when they're not actually doing any amplification?