[pkjames]

very exciting! can't wait to see the boards roll out!

 

[Alcaudon]

Very nice explanatory post runeight, thank you very much, I've appreciated it very much.......... can we, tube noobs, have more like this???

Btw........... is this project going to have a helping web page???

 

[fordgtlover]

Any thoughts on how the SOHA II stacks up in SQ against the Bijou.

 

[m0b1liz3]

Quote:

Originally Posted by Alcaudon /img/forum/go_quote.gif Very nice explanatory post runeight, thank you very much, I've appreciated it very much.......... can we, tube noobs, have more like this??? Btw........... is this project going to have a helping web page???

I would imagine that there will be one as soon as it is officially released.

Then there will be this forum too where people can discuss their builds.

 

[runeight]

Quote:

Originally Posted by fordgtlover /img/forum/go_quote.gif Any thoughts on how the SOHA II stacks up in SQ against the Bijou.

fordgtlover, only dBel84 has both amps at the moment. I'm going to be part of the SOHA II proto team and I will have them both when mine is built. Perhaps dBel84 will have time to comment on his SOHA II.

Alcaudon, m0b1liz3 is right on. There will be a website for the amp. The proto team has really helped to make website as good as we can when it goes public.

 

[dBel84]

my thoughts are slowly being compiled from illegible scribble..dB

 

[TimJo]

Quote:

Originally Posted by runeight /img/forum/go_quote.gif Alright, now it's time to talk about the input stage.

Thanks for the post runeight. I just printed it so I can study the circuit.

 

[runeight]

This post brings the input stage and the buffer together into a single channel amp.

You'll remember from the previous input stage post that the grid of the second triode is grounded. But this grid is an extraordinarily handy place to apply some negative feedback. We don't want a lot of feedback, but a little bit will be a good thing.

Here's the schematic:

The output is brought back to the second grid through a voltage divider (an extremely common and broadly applied technique). With the component values shown the NFB is only about 6db. This is not much NFB, but it has two really nice effects:

1. It extends the effective bandwidth of the amp, causing the high freq rolloff to be higher.

2. It almost takes control of the gain of the amp. Not quite completely because there is not much NFB, but mostly.

The second feature is important because you can roll different kinds of triodes into the front end, including triodes that have vastly different mu values. For example, the front end will support 12au7, 12at7, and 6922 (among others). The mu for these tubes is 17, 60, and 33 respectively. This means that the open loop gain of the amp will change dramatically when you swap these tubes in and out.

However, with the NFB the gain of the amp for these three different tubes is:

12au7 - 9
6922 - 10
12at7 - 13

Notice that the gain hardly changes. This means that when you roll these tubes you won't have widely gyrating volume pot issues. If you want slightly less gain, increase the value of R8 by some small amount.

So folks, there you have it. Assuming that the amp will work as well as it has in dBel84's prototype, this is about as flexible, low voltage, tube hybrid amp that you can make.

The power supply also has some upgraded features compared to the SOHA. The B+ supply uses a much better voltage doubler, for example. In fact, the SOHA PS will not work for this amp.

But now I think we wait for the prototype team to get their boards. If you're anxious to get things moving you can rag on JeffR at GlassJarAudio.

 

[Beefy]

I sort of understand some of that

You noted before that the power supply needs to be configured/tweaked with tube rolling. When do we get a sneak peak at that?

 

[runeight]

The PS will not need any active adjustment whether tubes are rolled or not. It's all fixed.

But, it needs to be better than the SOHA PS to support what this amp does.

I think we'll wait on the PS until the prototypers begin to prove out the amp design. Hang in there 'cause I'm anxious to get my boards and to start building too.

 

[Beefy]

Quote:

Originally Posted by runeight /img/forum/go_quote.gif The PS will not need any active adjustment whether tubes are rolled or not. It's all fixed.

Just re-reading back to your first post..... you don't need to adjust the heater voltage anymore? That is impressive!

 

[runeight]

LOL. You got me.

Yes, you will need to either change the heater jumpers or, if you wired a switch, throw the switch.

 

[dBel84]

Seeing as Alex has been asking me about my impressions and he has provided the guts of the circuit, I guess it is about time for me to compile some sense of my the few notes from my listening sessions.

I do not know exactly how many hours the amp has on it but I am sure it is going on 50 odd. There have been a few mods in between and I did not sit down and listen to all this music back to back. I did most of my listening with NOS 6080 tubes ( a 12AU7 variant ), it is not necessarily the best tube for this amp but after getting it all working correctly and testing the tube rolling ability, I defaulted to a tube that I was familiar with. There are noticeable differences between tubes so, as with the original design, this amp will be very sensitive to tube rolling which will afford people the ability to tweak the amp to suite there specific likes or dislikes. Some tubes had more bass but less control of it, the ampere 6922s for example have an incredibly liquid midrange while the Tungsol sound a little recessed. In fact, I started off listening with the buffers idling at 75mA and then turned them up to their designed operating point of 100mA, because I felt the mids were too recessed. Red label CD’s were the standard medium. Headphones were either K501 or DT770.

I listened to a lot of music over the course of evaluating this amp and I have listed a few of the tracks below. I was especially aware of the fact that this was a new amp and that I wanted it to sound good. I hope that I have been as objective as possible and I believe I have, something we have not mentioned on the thread is that I had a small noise issue and after much trouble shooting, narrowed it down to a very nifty feature that runeight had designed into the amp, unfortunately because of my findings, we have had to eliminate it.

Chesky ‘s Ultimate Demonstration Disc – this is a collection of music that has been recorded by Chesky Records to highlight specific aspects of music, it is a useful disc to get a feel for a systems strengths and weaknesses. Spanish Harlem by Rebecca Pidgeon begins with her singing unaccompanied and allows you to appreciate the sense of space and tonal accuracy of the recording. SOHA II allowed her voice to float centre stage, projected slightly forward and when the accompanying instruments join her, the subtle tonal changes of the rhythm shaker are clearly apparent. The instruments have their own stage, never intruding on Rebecca’s voice but not fading into the background either. This particular track made me very aware of how accurate the sound was being presented; no sugar-coated “tubeyness”, however, not as clinical as some SS amps I have heard that teeter edge on sibilance. Speaking of which, I find sibilance and female vocals to be a problem with some gear and I like to listen to Diana Krall’s “The Girl in the other room” and Katie Melua’s “Piece by piece” both of which have crisp vocal which has the ability to degenerate rapidly if the upper mids are overemphasized. They are both well presented with attention to acoustic instruments, sudden deep bass notes are fast and the music has a good sense of rhythm and pace. To test the amps ability to deal with bass, I listened to Stings “Desert Rose” which begins at subsonic levels and even had my AKG’s shaking. The transition to the opposite extreme of the frequency scale helps to highlight the top extention and the amps ability to represent the entire audible frequency spectrum. The other tracks which really give the bass a workout are the African drum albums from Chesky : Babatunde Olatunji - Love Drum Talk and Mapleshade Records – Asante. The bass is tonally accurate, fast and well controlled ie it was able to keep up with both drummers and retain the incredible dimension these musicians create without losing the sense of rhythm.

I am running out of steam here, a few more that really stand out. Jacquline DuPre’s rendition of Dvorak’s Cello Concerto – pure raw with emotion and the Cello’s sound is just breathtaking. YoYoMa can make it sing, but not with this degree of emotion. And while on the topic of expensive instruments which have age nurtured tone, Tao Ruspoli ‘s flamenco recordings for mapleshade are presented with such realism that you feel as though you are sitting on the wood floors in that old Baltimore house listening to him perform a personal show for you. Patricia Barbur’s raw vocals are also evident in her live Parisian recording. I like dramatic effects from time to time and Roger Waters Pros and Cons is no exception – the moment the drill enters the brain is mind numbing as the complexity of the sound is so clearly portrayed. Just as dramatic is the decrescendo in the penultimate movement of Giulini’s Beethoven 9th when there is an extended pause before the powerful tenor vocals make the hair follicles on the back of arms stand up or the sudden blast of a lonely piper on William Jackson’s Incholm. Rawness of a different genre is well portrayed in Jimi Hendrix’s bluesy “Hear my train a comin’ ” or the feedback from Stevie Ray Vaughn’s tube stack when he holds that incredible sustained note in “Little Wing”.

I have a few more scribbles but reading through what I have already written, it is clearly not objective, but then when is music objective. SOHA II just passes it through to the headphones uncluttered and with great authority.

I am enjoying listening to this amp very much. It does some things extremely well, in particular its authoritative bass reproduction. It separates instruments very well, perhaps not as well as some $2K amps I have heard but it performs well above it’s commercial price point and then some. I will leave the final word for others when they have built and tested theirs.

A further word of thanks to Alex for sharing his work even after all the adversary..dB

 

[fordgtlover]

dbel84

Thanks for your views. A new amp is always exciting. It seems thatthe SOHA II is shaping up to be a real treat.

I read a comment recently in the NorCal meet impressions that at least one person felt that the Bijou lacked bass. You comment above that the SOHA II has authoritative bass. Are you able to offer a comparision between the Bijou and the SOHA, and particularly in regards to bass reproduction.

 

[Cankin]

Any estimates of the cost of the kit or PCB board?