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Old 18th December 2008, 02:09 PM   #1
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Default DHT and speed issues

Hello there,
I have built my first amp these last weeks. It is base on 6SL7-2A3 (from jelabs design, with changes)
Learning from my mistakes, the amp now sounds very good (details, sweetness, harmonic richness, dynamics...) but...

...I had before that a class d amp (trends audio ta10.1) and the only thing i miss the former amp is its speed and transients. I find the tube amp slightly offbeat.

I thought SE DHT could also be renowned also for their speed.

From your experience, what would the key factor in SE DHT design concerning speed, before i go on buying (expensive) stuff ?

- cathode caps quality/value, input or output stages
- psu caps quality/value, first cap or following ones
- input/driver capability (mA) or swing (volts) or Zout (kohm)
- the tubes themselves, especially rectifier (i have 5y3 and gz34)
- signal cap
- other
In my experience, I think everything parallel (ie decoupling caps) creates trouble in music unity (time constants)
What improved speed once : cathode caps removed from 6sl7 but sound became gritty (impedance matching problem with high Zout 6sl7 ?)


My amp (attached):

every connection made with screw connectors to change everything easily until final design.
SOVTEK 2a3 (only 80% on plates because hard mistakes), BRIMAR NOS 6SL7 (and other brands), JAN-NOS-5Y3 or JJ-GZ34
i also tried 6sn7 but prefered 6sl7
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Old 18th December 2008, 02:10 PM   #2
SY is offline SY  United States
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What do you mean by "speed"? Bandwidth?

edit: A quick calculation shows that the Miller rolloff and the transformer rolloff won't be too far apart. That's not a good thing. You want to lower the driving impedance. Also, the driver stage will be panting trying to swing the volts to drive the output stage. I'd expect a lot of distortion.
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Old 18th December 2008, 03:56 PM   #3
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I don't mean a technical aspect (bandwidth...)
I just mean the musical fact of being with the beat, the contrary of a slow/dull perception of music.
I could say toe-tapping...
when I try to follow a rythm, i perceive the amp always being too late
eric
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Old 18th December 2008, 03:56 PM   #4
45 is offline 45  Italy
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Default Re: DHT and speed issues

Quote:
Originally posted by mahleriana
Hello there,
I have built my first amp these last weeks. It is base on 6SL7-2A3 (from jelabs design, with changes)
Learning from my mistakes, the amp now sounds very good (details, sweetness, harmonic richness, dynamics...) but...

...I had before that a class d amp (trends audio ta10.1) and the only thing i miss the former amp is its speed and transients. I find the tube amp slightly offbeat.

I thought SE DHT could also be renowned also for their speed.

From your experience, what would the key factor in SE DHT design concerning speed, before i go on buying (expensive) stuff ?

- cathode caps quality/value, input or output stages
- psu caps quality/value, first cap or following ones
- input/driver capability (mA) or swing (volts) or Zout (kohm)
- the tubes themselves, especially rectifier (i have 5y3 and gz34)
- signal cap
- other
In my experience, I think everything parallel (ie decoupling caps) creates trouble in music unity (time constants)
What improved speed once : cathode caps removed from 6sl7 but sound became gritty (impedance matching problem with high Zout 6sl7 ?)


My amp (attached):

every connection made with screw connectors to change everything easily until final design.
SOVTEK 2a3 (only 80% on plates because hard mistakes), BRIMAR NOS 6SL7 (and other brands), JAN-NOS-5Y3 or JJ-GZ34
i also tried 6sn7 but prefered 6sl7

IMHO it is the overall design. It is simple and works but you cannot ask more than that.
Careful matching with speakers could change a lot your sound. However this is not a cheap way to go, usually.

Maybe you can get a little improvement going to 100 uF for the cap close to the opt and increasing the cathode bypass caps.
Don't use old caps. Use new, freshly made capacitors. You don't need to buy exotic stuff.
In my experience good chinese 2A3's, like Golden Dragon or Billington brands, sound better than Sovtek.

If you wish to improve a lot your amp you need to go for another design.
I would start from the Audio Note Neiro or something like this: http://www.positive-feedback.com/Issue1/kismet.htm

Cheers,
45
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Old 18th December 2008, 04:13 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by mahleriana
I don't mean a technical aspect (bandwidth...)
I just mean the musical fact of being with the beat, the contrary of a slow/dull perception of music.
I could say toe-tapping...
when I try to follow a rythm, i perceive the amp always being too late
eric

My recipe includes at least 10mA of quiescent current in the preceding driver stage and fixed bias in the output stage. No electrolytics anywhere in the signal path.

Note that most large value motor run caps sound very "romantic" in the highs due to an appreciable amount of ESL/ESR - need to get some help using the Western Electric connection with higher quality film caps. (Can hum buck too. Search my older posts on this subject.) An unbypassed MRC is still better IMO than any electrolytic I have heard except a now virtually unobtainium Black Gate or Elna..

My current design uses 6SN7 based srpp driver running at 10mA and fixed bias on a 300B.. Some people might have an issue with this, but try it before you knock it. Note you need an additional gain stage ahead of the srpp unless you have a linestage with appreciable gain in which case with the 2A3 it would be optional..

Another very good option might be a choke or CCS loaded D3A (triode connected) or 5842/417A running at 15 - 20mA.. (More than sufficient swing to drive a 2A3.)

Note that the 6SL7 rp and rl conspires to produce a driver stage source impedance that is too high to effectively drive the miller capacitance of your 2A3.. (Even if such were not the case there is not enough current available to do the job.)

Look at my website for a design that uses the 6SL7 to drive the 45, it has a fairly low output impedance and does a nice job with the 45 and 2A3..
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Old 18th December 2008, 04:33 PM   #6
SY is offline SY  United States
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Quote:
a slow/dull perception of music.
Excessive even order distortion and probably too low damping for your speakers. Kevin's recommendations seem spot-on.
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Old 18th December 2008, 05:48 PM   #7
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I thought SE DHT could also be renowned also for their speed.>>

That's what I've found. But you don't have a DHT amp - you have an amp where the majority of the amplification is done by the 6SL7, which I find to have a rather thick romantic sound.

If you want the DHT sound build an all DHT amp.

andy
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Old 18th December 2008, 06:29 PM   #8
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I think it is not a problem from the speakers : TQWT designs of different fostex single drivers.

I sometimes read than simple anode follower stages are more musical than SRPP, mu followers etc (more "hifi") ...but i never other than anode-f.
Anyway my philosophy is often the simpler the better... Fi X or Decware Zen made me desire to build my own amp... I intend to go Direct Coupled when i will be more sure of my design.
Anyway the slow aspect is just slight, but enough to bother me.
perhaps a separate anode loaded input+driver ?

eric
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Old 18th December 2008, 06:29 PM   #9
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for example ?
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Old 18th December 2008, 06:34 PM   #10
SY is offline SY  United States
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Quote:
I sometimes read than simple anode follower stages are more musical than SRPP, mu followers etc (more "hifi") ...
You need to change what you read. People make all sorts of silly unsupported pronouncements. That doesn't make them true.
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