[ebag4]

I am currently running a Bottlehead Stereomour (2A3, 3.5 wpc).  My previous speakers were 97dB efficient, my new speakers are a little less efficient at 94dB.  The amp is high passed at about 175Hz, the speakers are an 8 ohm load, so easy to drive (dual 12" servo woofers per side cover to 200Hz, have their own amp).

My issue is that on a small portion of music the level is maxed out with the Tortuga level control pegged.  It still sounds great but I don't believe I am getting everything these speakers offer.  They sound great on 90% of my music at levels I typically listen at, but every once in a while they need a bit more.

I have tried a small digital amp (Yuang Ging Blue Black board?) that puts out about 15 watts, but it sounded brittle compared to the 2A3 although the additional power offered what is needed.  Ideally there would be a way to get more power out of my current amp, but that doesn't appear to be possible.

I am considering buying or building another tube amp.  I would like recommendations as to what amp would maintain the 2A3 sound but offer something approaching 15 wpc.  It must be quiet.  I considered the Bottlehead 300b amp, but reading up on it I don't know if I would like the sound.  I would like to keep my cost under $1000 if possible.

Thanks,
Ed

[Quiet Earth]

Hi ebag4,

I think your problem is one of insufficient gain, not power. Your amp appears to have a single gain stage driving the 2A3, and your Dac looks like it is a solid state design with a standard 2V out. Throw in a passive volume control and you have all of the symptoms of insufficient gain. Your amp is probably not being driven to its potential. In other words you probably aren't getting the ful 4 watts out of it.

I think you either need another gain stage in the amp or an active preamp. Can you put a 12AX7 in the Bottlhead instead of the 12AT7? That would give you a little more gain.

[opnly bafld]

deleted

[Quiet Earth]

I think the specs are a bit optimistic. And even if it all looks good on paper, there is only so much a single gain stage amp can do. IME, passive volume controls only work well with very high gain power amps and very efficient speakers. For a tube amp to be high gain, you need to see two triode sections before the output tube.

[Quiet Earth]

Also, just because a DAC can put out 2V,,, that doesn't mean it will always put out 2V. It means  that it puts out 2V at full scale digital 0, which is your loudest peak before digital overload.

What happens when you play a quiter recording? Lower output. So you can't just look at the amp sensitivity and think all is well if it is under a volt. Hope that makes sense.....

[opnly bafld]

deleted

[ebag4]

Hi guys, thanks for the input. 

I need to update my systems page, it is now:

Mac Mini w/external firewire drive>Geek Out USB DAC ("soon" to be Geek Pulse Sfi)>Tortuga LDR Volume Control>Bottlehead Steremour>GR Wedgie (94dB Efficient),  Tortuga also feeds 2 servo sub amps.

To test whether I am losing output due to the passive volume control I bypassed the Tortuga completely and sent the signal to the amp directly from the DAC using digital volume control.  In this very unscientific test I found no noticeable loss in volume, in both instances the volume controls were ramped up to maximum output.

The gain question is interesting.  I have searched and verified tonight that the 12AXT is not a direct swap, although there were a couple of ideas on how you might go about modifying the amp to work with it, but nothing more than theory.

I will likely wait until my Geek Pulse DAC arrives before doing anything more, it could be that it's output will help with the maximum volume issue.

Any and all ideas are appreciated.  After I wrote the initial post I started googling PP 2a3 amps, it seems there are a few out there.  That may be a path to look at if my gain issue can't be addressed in another way.

Thanks,
Ed

[Quiet Earth]

A 12AX7 and 12AT7 have the exact same pin-out and they draw the same heater current. A lot of guitar players will put a 12AT7 in place of a 12Ax7 to lower their gain, so I figured why not try the opposite to raise your gain. Perhaps there is something unique to the Stereomour that it cannot use a 12AX7. I am not familiar with that amp. It probably would not be a final solution anyway.

I think that asking a DAC to drive a two stage power amp plus an electronic crossover is asking a lot. It seems like an active preamp with low output impedance would do a better job. You did a very nice job on that 2A3 amp and I think you should keep using it. 

Opnly bafld,
You didn't have to delete the point about the 0.4V sensitivity spec. It was an excellent observation. 

[ebag4]

I just remembered I have an old outlaw 950 boxed up in the closet.  I will try putting that in front of the amp in place of the Tortuga.

[opnly bafld]

Mac Mini w/external firewire drive>Geek Out USB DAC ("soon" to be Geek Pulse Sfi)>Tortuga LDR Volume Control>Bottlehead Steremour>GR Wedgie (94dB Efficient),  Tortuga also feeds 2 servo sub amps.

You're using a dac/headphone amp to drive your speaker amp? 

[ebag4]

You're using a dac/headphone amp to drive your speaker amp? 

Yes, it has a line out.  It drove my V1s using the same amp as well or better than my Tranquility SE but to be fair I did not have the Mac Mini at that time.

[opnly bafld]

Yes, it has a line out. 

Good, looking at pictures all that I saw was h/p outputs. 

Do you have any specs on it?
Maybe it is not a full 2v out.

[ebag4]

These are the specs to the unit I am running.

450mW, 2.65vrms (Geek Out 450)
◾Output impedance: 0.47 ohms (headphones) and 4.7 ohms (line out)

[Quiet Earth]

Ed,

It's quite possible that your amp doesn't have enough power for your new speakers. Let's face it, 4 watts is 4 watts. I just thought it would be a good idea to investigate the gain structure before you give up on your beautiful 2A3 amp. A two stage amp (gain+power) is typically asking for a pre-amp. A nice pre-amp could make your amplifier sound much bigger than it has a right to. Thought it was worth a shot.

The fact that you use a headphone/Dac as your source kind of muddies the water for trouble shooting. I think it will be hit or miss, and ultimately up to you to decide what works and what doesn't. Try the outlaw to see if it makes your amp deliver more power, and also try a 12AX7 in place of the 12AT7 to see if you get a little more gain. At least you will know a little more about gain structure and puting it all together.

[ebag4]

I was able to insert my Integra DTC-9.8 pre-pro into my 2 channel system today and take some dB measurements. 

With the Integra set up flat (0 dB on all settings) and the volume at 0dB the readings were very similar to the maximum volume on my Tortuga LDR given the same material. 

For most material 2-3 +dB is the loudest I would ever listen.  For one of the tracks recorded at the lowest level a +10dB setting is the loudest I could ever listen.

I am thinking I need to add a preamp.  The Integra sounds good and adds what I perceive as drive, especially in the bass frequencies, but given the same volumes the LDR is cleaner IMO.

So I am looking for recommendations.  If a preamp, which one.  Staying with Bottlehead I could go with The Smash (10dB gain at 1kHz), would it hold the Stereomour back?  If pressed I could go with the BeePree (8.3dB gain at 1kHz).  Not sure if I will like the 300b, I have never heard one.

Bottom line is I am happy I can stay with my Stereomour, although I would listen to argument recommending forgoing the preamp and going with a 300b or other amp instead.

Thanks,
Ed

[opnly bafld]

My concern would be that the added gain is driving the amp more into clipping and hence it sounds louder, but is also causing it to be less clean than with the LDR. The drive you mention is what many like about preamps with gain, but unfortunately they don't make the amp have more watts. 


FWIW ~ YMMV ~ JAIMWG*

*just another idiot making wild guesses

I don't listen very loud and use a 1.8 watt integrated to drive 94.5 dB 1w/1m speakers.

[ebag4]

Thanks OB, that is a concern of mine as well.  I believe the fact that the 2a3 is high passed around 175Hz should make it less likely to clip, but I am not certain I know what clipping sounds like. 

The sound was not bad with the Integra, just not as right as the 2a3 with the LDR.

Best,
Ed

[opnly bafld]

What is a GR Wedgie?
Do you know if they are rated at 1w/1m or 2.83v/1m?
(I think Danny usually rates his speakers at 1w/1m  )

I believe the fact that the 2a3 is high passed around 175Hz should make it less likely to clip, but I am not certain I know what clipping sounds like.

True

[ebag4]

GR Wedgie








I believe Danny uses 1W/1M for measurements.

I am not certain I know what clipping sounds like, but am I correct to understand that if I can keep turning the volume up (on the Integra in this case) and the volume continues to increase when reading the SPLs on a dB meter that it is unlikely I am clipping?  I am able to do that all the way to +18 dB on the Intregra, although I don't think I could ever use it beyond +10, it gets too loud.

One other question, does a preamp with 10dB gain to equate to +10dB on the Integra volume control?  It seems it should however I am not certain what the relationship is between preamp gain and volume control settings.

Thanks,
Ed

[avahifi]

A few observations regarding tube gain and circuit gain that seems to not be widely understood.

In a typical audio circuit, the closed loop gain of the circuit is set by the feedback loop and is independent of the actual gain of the tubes used.  Since every single tube is a bit different, this is a great way to keep the circuit performance consistent as tubes age and change gain and to keep the channel balance identical independently of the brand and type of tubes used.

So changing from a lower gain tube, such as a 12AU7, to a very high gain tube, such as a 12AX7 tube, won’t change the overall circuit gain at all, it will only change the amount of feedback, as the feedback is the difference between open loop gain of the tube and the closed loop gain fixed by the gain set resistors in the feedback loop.

Of course if the gain of the tube selected is too low for the application, then circuit will run out of feedback, resulting in greatly raising harmonic distortion and very muddy sound.

A good example of this is trying to do an RIAA phono preamp using only a single dual triode tube for amplification.  The RIAA equalization requires 40 dB of gain at 1k Hz with about 20 dB less gain at high frequencies and 20 dB more gain at low frequencies, total 60 dB gain required at 20 Hz or so.  Unfortunately, the highest gain dual triode tube available, the 12AX7, maxes out at 60 dB of open loop gain. Thus any circuit  configured to attempt this runs out of feedback at low frequencies, won’t follow the EQ curve, and always will provide colored and non-linear output.

Using a tube type with too much gain for a given circuit will cause problems too.  The tube will cause too much feedback for the circuit parameters and result in TIM (transient intermodulation distortion) and a hard, bright, nasty musical presentation.

Much efforts at “tube rolling” is simply trying to find the sweet spot of gain for your particular tube selection to minimize both low frequency THD and high frequency TIM.  Its much easier to do this with a tube tester and knowledge of the way your circuit is actually working.  Otherwise its back to tire testing without a pressure gauge and wheel balancer.

Preamp overall gain is set by the fixed gain of the circuit as modified by the volume control.  Note that since different volume control designs vary in their “taper”, (the relationship between their rotation and their actual attenuation), where the control is actually set is not consistent between  brand and/or design.

For example, a typical AVA preamp with 20dB of gain in the line circuit and a log taper precision volume control, unity gain is at about one half of total volume control rotation.  Below this the volume control is actually reducing source gain, above this increasing source gain.

Where the volume control will actually be set in your system depends on the output level of your source music, source gain of your DAC, tuner, phono preamp, or tape deck (all usually different and sometimes widely different).  This also is dependent upon the gain of your power amplifier, the sensitivity of your speakers, and finally the acoustics of your listening room.

If you are trying to run a system without a preamp (often DAC only) then you are dependent upon having adequate output output from the DAC, adequate gain from your power amp, reasonable speaker sensitivity, and perhaps a rather “hot” listening room.  You still might be out of luck with many CDs produced at a rather low output level.  Without a preamp you can only turn the music level down, not up. This will work for some of you. but not all.  You really need to be careful of your choice of system components to make this work well.

Note once again that changing to higher gain tubes won’t help, this generally will only change the distortion characteristics of your system, for better or for worse

Good luck, and have a great new year.

Frank Van Alstine