| trifidmaster |
Has anybody built one like this?
(with EL84, 6922, 5U4G)
trif. |
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| Eli Duttman |
The Dyna ST35 should give you a reasonable starting point.
Dyna ST35 schematic here. |
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| trifidmaster |
Eli Duttman,
Thanks for the schematics.
BTW: I have never built any tube amp, but a colleague of mine borowed me his modified Dyna and Lafayette amp.
I liked the Dyna, but it did not have nice highs.
trif.
Note: I have auditioned those amps in my listening room/studio. That room is acoutically treated (with many acoutic panels and long soffits) and the freq response in it is extremely flat and the decay times are well under control. |
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| ErikdeBest |
Hi Trifidmaster
The 3rd edition of Valve amplifiers, by Morgan Jones, shows a schematic with the tubes and circuit you want: a ECC88 as driver/phase splitter (concertina) and 2 EL84's at the output.
Erik |
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| Wavebourn |
| quote: | Originally posted by trifidmaster
[B]Eli Duttman,
I liked the Dyna, but it did not have nice highs.
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It does not depend on the schematic which is classical in this particular case. With different output transformers you'll get very different results.
However, I would decrease impedance of the feedback loop 10 times splitting cathode resistor by 1.2K bypassed by a capacitor and 100 Ohm in series, 27K resistor in the feedback would be reduced to 2.4K. Correspondingly, the capacitance in parallel with this resistor will be increased until the amp get stable on highs. Or, you may leave the cathode resistor as is, connect to the cathode one 100 Ohm resistor with 1000 microFarad capacitor in series, negative leg of the cap is grounded, and feedback will go to the point of interconnection of the 100 Ohm resistor and the capacitor.
It will decrease a local feedback in the 1'st stage increasing gain; the overall feedback will bne deeper, and non-lineariries in the feedback loop will be reduced many times.
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| Wavebourn |
| quote: | Originally posted by Wavebourn
It does not depend on the schematic which is classical in this particular case. With different output transformers you'll get very different results.
However, I would decrease impedance of the feedback loop 10 times splitting cathode resistor by 1.2K bypassed by a capacitor and 100 Ohm in series, 27K resistor in the feedback would be reduced to 2.4K. Correspondingly, the capacitance in parallel with this resistor will be increased until the amp get stable on highs. Or, you may leave the cathode resistor as is, connect to the cathode one 100 Ohm resistor with 1000 microFarad capacitor in series, negative leg of the cap is grounded, and feedback will go to the point of interconnection of the 100 Ohm resistor and the capacitor.
It will decrease a local feedback in the 1'st stage increasing gain; the overall feedback will bne deeper, and non-lineariries in the feedback loop will be reduced many times.
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Oops... Sorry for the mistake, feedback will go to the cathode, of course!
One more way to reduce distortions is to use the amp in inverting mode when both input and feedback is connected to the same grid, but it is good when you are driving the amp from low and known impedance. Cathode resistor in such case should be bypased by a cap, and output transformer taps must be inverted.
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| Brian Beck |
| quote: | Originally posted by Eli Duttman
The Dyna ST35 schematic here. |
Just a note: I've seen that schematic floating around the web. It was redrawn in error, leaving out the 60uF section of the power supply cap, which should be connected from the HV rectifier diodes to ground. This page has a scan of the original, although details are hard to read.
Dynaco page |
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| trifidmaster |
Dear gens!
Thank you very much for your comments!!!
:)
..this is getting exiting...
I have the 3rd edition of Valve amplifiers, by Morgan Jones book.
He uses the ECC88, indeed.
Is the ECC88 replacable with the 6922?
I already have the Lundahl trafos, they look very cool - I hope they will sound warm, too.
I also have the EL84 tubes/sovtek, matched.
And the 6922s are with me too.
The power supply will be with chookes, and tube diode.
BTW: the enclosure is ready (no wood-cover/frame work yet).
I have used all the advices for my enclosure from Morgan Jones-building Valve Amps.
LOTS of work, but I think the end result looks nice.
trif. |
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| trifidmaster |
Here is the materail layout and the "dry" material assembly.
As you can see the left/right plates are made from U-profile.
The front/back/bottom/top plates are made from 3 mm thick brushed and anodized aluminum plates (the top plate is not on the photo).
On the left side you can see a "smaller" plate called Power trafo plate.
The purpose of this plate is visible from the assembly picture.
trif. |
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| trifidmaster |
| And the "Dry" assembly: |
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| trifidmaster |
And finally the assembled enclosure. No inside electrical work is yet done. The power trafo is big, and will be on the top of the enclosure.
Note: the choke(s) (1 choke/channel) and the output trafos are inside the enclosure - I have indicated the fixation holes for these trafos.
On the front panel the hole for the potmeter is still missing.
trif.;) |
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| trifidmaster |
| ...and the back panel: |
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| trifidmaster |
| I have put isolation around the cut for the power transformer |
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| trifidmaster |
And here is the inner view of the enclosure,
the power trasnformer is already installed. |
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| trifidmaster |
The enclosure with the power trafo.
It is heavy.
About the topology:
the end-part of the amp will be like the BV, the splitter will be made by the 6922, but the front end will be different (also made by another 6922).
trif.
;) |
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| karma |
looks good;)
any wood on the sides of the case? |
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| trifidmaster |
Yes, wood will be added on the sides and on the front, not on the back, like this:
I hope my design is not too dense.
trif. |
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| karma |
no its going to look good.
:) |
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| trifidmaster |
Karma,
Thanks for the good words.
I wanted to have an ellongated design, so the top plate is 400mmx220mm.
I have made sure that the center of the tubes are 50mm from each other. As I wrote, I will use 1 choke/channel (the chokes are also nicely visible in the "inner" picture).
trif. |
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| trifidmaster |
The HT will be hybrid, the filaments will be DC driven.
Meantime, the Fostex FE127 single drives have arived. I want to use this amp with the Fostex speakers. Well, it is kind of "late" question, but is the BV sufficient for the Fostex speakers?
trif.
;) |
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| trifidmaster |
I have noticed that the screews used to keep together the choke plates is not fully tighted.
Is this normal for a choke?
trif. |
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| Wavebourn |
| quote: | Originally posted by trifidmaster
I have noticed that the screews used to keep together the choke plates is not fully tighted.
Is this normal for a choke?
trif. |
You may tighten them. Also, if you don't intend to rewind your choke one day you may coate it by epoxy. |
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| Tweeker |
| PP EL84s should be fine for FE127s. Fostex claims 91dB/watt. |
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| trifidmaster |
How much time is needed for "burn in" the tubes?
trif. |
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| Wavebourn |
| quote: | Originally posted by trifidmaster
How much time is needed for "burn in" the tubes?
trif. |
It depends on a post-hypnotic suggestion. |
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| trifidmaster |
| Uhhh, now I am lost...can you elaborate more? |
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| trifidmaster |
The electronics is done by my colleague.
The amp is evolving and I am learning a lot...from him.
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| Wavebourn |
| quote: | Originally posted by trifidmaster
Uhhh, now I am lost...can you elaborate more? |
Break-in is a well known effect in the Hi-End business.
Trance state induction is needed, it means good rapport and tight feedback between a person who sells and a person who buys, in such case suggestions works well and break-in happens exactly as predicted, with the greatest satisfaction. Tubes, cables, amplifiers, and even whole systems may be breaked-in to the best satisfaction of the owner. Sometimes, fellow audiofiles and articles interfere, so break-in happens at a different time, and while listening to a different music.
Sorry if I misunderstood your question. |
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| Wavebourn |
| quote: | Originally posted by trifidmaster
The electronics is done by my colleague.
The amp is evolving and I am learning a lot...from him.
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If I see well, you have only one ground wire going to each filter capacitors. I'll suggest you to wire them step by step, as shown on the schematics, from transformer and rectifier to the cap, from the cap to the chocke to another cap together with the ground wire, and so on. Ground wire is actually very loong in terms of big currents and small signals, so ground near the rectifier and/or power transformer is completely different ground wire than near the input socket! |
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| trifidmaster |
The grond wire is 2.5 mm2. We will post another picture where the grounding is more clearly visible. OK?
Thomas and trif. |
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| trifidmaster |
Yesterday Thomas could finish the building of the amp, and
we turned ON the amp - BIG MOMENT.
Note: No GLOBAL feedback is implemented yet.
Both channels worked! :D :D
Thomas checked the voltages, and all values were as planned.
And we started to listen to the amp with several CDs,
and at the certain moment we have observed CLIPPING - the input stage was CLIPPING.
Paralel to the cathode rezistor one mkp and one elko cap were mounted.
We have measured with the scope the signal - coming from the CD player (Philips CDR570) - and it had 2.8 V high peaks
Note: we do not know if this is normal for a CD player.
The plate current of the input stage at this point was 14.5 mA and the grid voltage was slightly negative but surely no enough for such a high input signal.
So, we have changed the cathode and plate rezistors, and the plate current became 4.5 mA, and the grid voltgae became -2.4 V. We have removed the cathode capacitors/per channel.
Certainly the clippind was reduced - but still audible with some CDs.
BUT the sound became completley unacceptable: it was more tight, no spaciness, kind of dry and woody sound. Installing back the cathode capacitorss the sond became more acceptabe, but still not good.
Finally we put back the original rezistors for the plate and the cathode.
I have learned: more inputstage anode current has a nicer sound, and the cathode capacitors have also nice effect on the sound.
Lower plate current has a tighter sound. The input stage has VERY BIG effect on the sound.
Any suggestion(s)???
Thoams and trif. |
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| trifidmaster |
At this moment I like to hear from you gens what do you think about my observations.
Actually, I am used to listen to my studio monitors - Dynaudio 2.1 system what just sound very good for many different type of music. The frequency response in my room is +-3dB. So, I have something good to compare my tube amp.
But I am not that stage yet, since the sound what comes out from my PP tube amp is far from what I like to have.
Why is the higher plate current better sounding compared to the lower one? (14.5 mA compared to 4.5 mA)?
Why the caps installed paralell on the input stage cathode rezistor sound better?
We have another issue - to be solved:
As I wrote earlier, the rectifier is hybrid and made by an excellent 5U4G. Very nicely looking tube.
Turning on the amp, this tube kicks in faster (in about 5-8 secs.) compared to the EL84s.
Without any protection the voltage on the rectifier cap could evenutally go high (over or very cloose to the voltage rating of the rectifier cap=450V), therefore we plane to implement a delay for the heater of the 5u4G. Sure, I could go for another tube diode (what kicks in slower), but for the time beeing I like very much the look of this beauty=5U4G.
For the moment we have put a power rezistor between the HV and ground so that we have already a consumption of the HV before the EL84 s kicks in and the HV never goes too high.
trif. |
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| trifidmaster |
Here is the promised inside picture.
Wavebourn, can you elaborate more on your grounding wire comment (few post earlier in this thread)?
trif. |
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| trifidmaster |
and the "final" (no frame work yet) look:
trif |
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| Wavebourn |
Looks nice!
But if you like the 2'nd harmonic it means you like lower resistance load of a triode, it not necessary means higher current. |
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| trifidmaster |
Wavebourn,
How do you know that I like 2nd harmonics?
Please tell me more, I am now very curious.
trif. |
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| Wavebourn |
| quote: | Originally posted by trifidmaster
Wavebourn,
How do you know that I like 2nd harmonics?
Please tell me more, I am now very curious.
trif. |
Correlation between schematics, oscillograms, and sounds in imagination. No miracles, just an experience. :cool: |
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| trifidmaster |
| quote: | Originally posted by Wavebourn
Looks nice!
But if you like the 2'nd harmonic it means you like lower resistance load of a triode, it not necessary means higher current. |
Are you sure? Is it not the opposite way? |
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| trifidmaster |
| quote: | Originally posted by Wavebourn
It does not depend on the schematic which is classical in this particular case. With different output transformers you'll get very different results.
However, I would decrease impedance of the feedback loop 10 times splitting cathode resistor by 1.2K bypassed by a capacitor and 100 Ohm in series, 27K resistor in the feedback would be reduced to 2.4K. Correspondingly, the capacitance in parallel with this resistor will be increased until the amp get stable on highs. Or, you may leave the cathode resistor as is, connect to the cathode one 100 Ohm resistor with 1000 microFarad capacitor in series, negative leg of the cap is grounded, and feedback will go to the point of interconnection of the 100 Ohm resistor and the capacitor.
It will decrease a local feedback in the 1'st stage increasing gain; the overall feedback will bne deeper, and non-lineariries in the feedback loop will be reduced many times.
|
Wavebourn,
If you read my very 1st post in this thread, you can see that I am using the 6922 tube. This tube typically uses 80-100 Ohm cathode resistor (I have 100 Ohm cathode resistor). Now, how can I split this 100 Ohm into 1.2K and 100 Ohm in series?
trif. |
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| trifidmaster |
Have I asked soemthing stupid?
tirf.
| quote: | Originally posted by trifidmaster
Wavebourn,
If you read my very 1st post in this thread, you can see that I am using the 6922 tube. This tube typically uses 80-100 Ohm cathode resistor (I have 100 Ohm cathode resistor). Now, how can I split this 100 Ohm into 1.2K and 100 Ohm in series?
trif. |
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| trifidmaster |
This post will be a bit long. Here it comes.
I have reported that we had "clipping" of the 1st stage.
This clipping was auible only at very high volumes of low frequency music (bass notes)...
Well, that was a wrong conclusion. (by the way, because that wrong conclusion I have spent hous studiyng the input stage).
Thomas an me started to measure with a scope further the amp, and the we have found very sharp peaks were alos present (high low frequency music) on the high voltage power supply.
So our attention went to the choke, and we have bypassed the choke with a resistor. The peaks were still there.
And at that moment our attention turned to the output stage.
Thomas went deeply into investigation, and he detected a very high frequency oscillation (during the high volume low frequency music), above 1 mHz. He has measured kind of envelope(s) with high frequency content.
At that moment the grid stopper was 330 ohm, and the ultralinear resistor was also 330 Ohm.
We have changed the grid stopper to 4k7, and the ultralinear also to 4k7. The nasty oscillation nearly disappeared.
I say nearly, bacause at the highest volumes, we still could detect a very small clicks in the music.
We need help. What to do, how to proceed further.
trif. |
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| SY |
| Just for fun, watch what happens to the output stage bias when driven at high volumes. I wonder if the clicking you hear is actually blocking. |
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| trifidmaster |
We see a dip what is full high frequencies...
Thomas and trif. |
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| SY |
| Um, I'm not sure I understand that. Can you elaborate? |
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| trifidmaster |
The cathode is biased with a 270 Ohms resistor (and decoupled), there’s 10V DC on it.
But when the spike happens, this voltage suddenly drops to 8V and then come back to 10V with a kind of RC charge shape.
In fact by zooming with the scope on this effect, we see that it is a kind of bubble full of high frequencies (approx 1MHz).
But that happens only at very high levels…
..and we do not see a dip on the grid |
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| trifidmaster |
Now we have on the EL84s 10k grid stoppers,
270 ohm catode rezistors with 220u and 0.82 u caps in paralel, the Ultra linear is via 100 ohm. The EL84 grid leak is 100k.
The cap between the splitter and the output stage is 330nF.
The concertina is with 10k anode, cathode, and grid stopper resistors.
The input stage is with 22 k grid stopper, 100 ohm catode rezistor with 220u and 0.82u caps , 15 k anode resistor, and 100k grid leak.
This means we have changed quite a lot of the resistor values.
And we STILL have that strange spike sound at higher volumes.
We have also used TRIANGLE signal to see/hear what happens.
With triangle input signal, at low volumes we have NO spiky sound, at mid potmeter position we have LOTS OF (repetitive) spiky sound, and surprisingly at top potmeter position NOW spiky sound, so it seams to us only in the mid position of the potmeter we have the spiky sound.
Gens, do you have ideas?
Please help.
Thomas and trif. |
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| trifidmaster |
Further info: the HT rectifier is hybrid
the heater PSU for the tubes is DC
Removing one concertina does not affect the unwanted spiking/clipping in the other channel
Removing concertina + preamp tube from one channel does not help either - the effect is a bit smaller
the concertina consumes 9 mA
the EL84s consume just below 40 mA
the input stage needs 14 mA
the power trafo is rated 700mA for the HT
changing CD player does not help
disconnecting one input does not help
Now I am lost...:bawling:
What is wrong?
Do I have to disassemble the amp and make another topology???
trif. |
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| trifidmaster |
| The cure: one tek scope's probe connected to the amp's 0 V wire. :D |
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| Wavebourn |
| quote: | Originally posted by trifidmaster
The cure: one tek scope's probe connected to the amp's 0 V wire. :D |
Most probably some ground loop causes oscillations. |
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| trifidmaster |
| ..or connecting the amp's 0 V wire to the amp's chassis also helps. |
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| Wavebourn |
| quote: | Originally posted by trifidmaster
..or connecting the amp's 0 V wire to the amp's chassis also helps. |
It means you forgot to ground it at all. :D |
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| trifidmaster |
Why it is necessary to connect the amp's 0 V wire to the amp's chassis?
Is there a leak?
Can you gens explain?
Note: if the chassis is connected or not to the saftey earth - what IS in normal case - does not make any difference.
Imagine that the chassis is made from wood/plastic. |
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| trifidmaster |
I am a physicist.
Saying/writing that the amp's 0 v/ground has to be connected to the chassis to avoid oscillations, or saying because it has to be like that...DOES not give me an acceptable explanation. This sounds like kind of magic.
So, is here anybody who can explain in a rigorous way why it is necessary to connect he amp's 0 ground to the chassis?
I am not asking about the safety reason, that I know.
trif. |
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| planet10 |
| quote: | Originally posted by trifidmaster
So, is here anybody who can explain in a rigorous way why it is necessary to connect he amp's 0 ground to the chassis? |
I believe it has to do with safety.
dave |
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| trifidmaster |
Dave,
Thanks for your answer. Your answer seams to me also the most logic one=safety.
I just can not accept that if the amp oscillates, it is due to the missing connection between the amp's "OV" ground and the chassis.
Maybe in that case the amp will "brumm" or "buzz" but it should not oscillate.
In the meantime my amp had 4 variations.
Stefan |
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| EC8010 |
| quote: | Originally posted by trifidmaster
So, is there anybody who can explain in a rigorous way why it is necessary to connect the amp's 0V ground to the chassis? |
Stray capacitance. There is inevitably capacitance between each and every part of the circuit and the chassis. Thus, there are two capacitors in series that connect the output of the (non-inverting) amplifier back to the input, potentially causing oscillation. Connect the chassis to 0V and you have shorted the junction of those two capacitors to 0V and broken the positive feedback loop. |
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| trifidmaster |
And what is the value of the stray capacitance(s)?
The calculated value is a few pico.
What frequency would give this value (a few pico) with 8 ohm output, and 25 kohm input? That is far from audible frequency, isn't it?
Stefan. |
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| trifidmaster |
Small update:
the "1st" version of my amp is ready - after lots of experimentation.
Now it is in "current injected" version - and it sounds VERY GOOD.
Soon I will check the amp with a network analyzer.
Note: the 6922s are so beautiful, they just need to be driven with the right plate current.
trif. |
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| trifidmaster |
and the pic of the "1st" version:
Stefan. |
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| trifidmaster |
Update:
After applying feedback, the max amplification became not enough.
Originally the preamp and the splitter were in 2 separate envelopes/channel.
Now, in the 2nd topology the the preamp + splitter are moved into one envelope. Doing so, one full tube (6922) became free/per channel. And as a next step this free tube was used as extra preamp + current source, and the amplifiaction is now more than enough.
Playing with the feedback (feedback resistor value + feedback topology) the sound starts to be "acceptable" with many different music including classical music.
The BEAUTIFUL sound of the 6922 is audible.
Stefan. |
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| trifidmaster |
And finally the speakers are ready for the amp.
The sound is very "impressive". The speakers are in closed box, and still bass comes out from them. Highly recommended!
trif. |
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| trifidmaster |
Here is the enclosure design - currently in "production"
trif. |
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