| Shoog |
I have been working on this preamp for about 2 years now. The latest alterations have produced a significant leap in performance so I thought I would share it.
The whole project started with the Vacuum State Valve Buffered Inverted Gainclone. I built the vary basic version as described in Joe Rasmussens article. I decided to try to emulate his more sophisticated version with a Valve CCS and a MOSFET constant voltage. As explained in Joes original article, by keeping the output valve at a constant current and at a constant voltage we get all the benefits of a Cathode follower but without its main disadvantage,ie 100% cathode feedback - which is widely recongnised as robbing the sound of life and punch. I built a few versions of this at various voltages and was happy with the result - enough so that when I decomissioned the Gainclone I kept the buffer on as my main preamp.
However it had one main disadvantage and one minor one. The main one was that as a cathode follower it was unity gain which wasn't quite enough for my power amp. The minor disadvantage is that it was a fraction bass shy.
I then went back to the vacuum state website and looked at the FVP5. This inspired me to convert my split rail buffer into a single supply preamp and to add a voltage gainstage. This was a big improvement, with extra gain and increased bass punch.
I then got involved in a thread about LED bias for valves and introduced it for the output stage. This was a help. I then tried a Green LED on the cathode of the voltage Gainstage. This produced a soft and smudged sound which was due to the low current through the LED of only 4mA. Further discussion suggested apply supplementary current to bias up the LED into a more linear range. This I did with a little circuit off the heater supply. This was a signicant improvement over the unboosted LED or an unbypassed resistor.
This final design is very detailed with a very natural top end and a powerfull bass. Its only disadvantage is the it has a little bit to much gain. The voltage gain valve could usefully be replaced with a lower MU tube for a lower gain overall. The only thing to consider is that the voltage on the grid of the second stage needs to be half the +B voltage plus a few volts.
It only remains to thank Joe Rasmussen for the original inspiration and the fundamentals of the circuit design. I also would like to thank SY for his ideas and help on LED bias.
Shoog |
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| Andypairo |
Nice one, but isn't the IRF510 a too low voltage device?
I see it just has a few volts to drop but I wonder if it could get into troubles at power up/down.
Cheers
Andrea |
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| Shoog |
"Nice one, but isn't the IRF510 a too low voltage device?"
I've been running mine for over a year and had no issues so far.
Shoog |
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| Shoog |
I have made a slight modification since posting the original circuit. I have changed the 1K resistor, which supplies the anode of the first stage, to a 660R. This is because the 1K produced a slightly bloated bass sound. As I said in the circuit, I suggest using a 1K5 pot to tune this to your exact taste. You can achieve a very full on hard sound or a very smooth sound by slight variation of this value.
Shoog |
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| Andypairo |
Thanks for the reply,
Iìve read that to sing this amp needs a good power supply, IIRC Allen Wright uses a super-regulator.
What did you use?
Cheers
Andrea |
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| maikel |
Hi,
In my opinion is the superregged PSU form the original design from Allen a balanced PSU, as on the other hand this schema uses a unbalanced PSU with a different value of 156V DC
Is it therefore dificult to compare thes PSU's ??
Greetings
Maikel
PS: spare me, I'm a newbie :-) |
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| Shoog |
The power supply I used was a simple 5 stage CRCRCRC filter. IT is said in the original design that the superreg is central to the sound of the circuit.I cannot comment as I have never heard the superreg.
However in my small experience the simple RC filters often sound superior to regulated supplies.
The 156V supply can be varied but you will have to adjust the circuit to compensate. This shouldn't be to difficult.
Shoog |
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| Andypairo |
Hi Shoog,
I made some calculations of the working point of the tubes, assuming 2V drop for the red led the SLCF works at roughly 5mA and slightly less than 80V, right?
Inspired by this thread I cooked up my version of the tube buffer you proposed time ago, using asymmetrical split supply (+100 and -50V), a C4S current source (instead of the lower tube) and a IRFD110 Mosfet for the constant voltage bootstrap.
Tested it on my scrap speakers and, tough I can't comment about the sound, it seems to work quite well.
The simple cathode follower was surely worse.
Thanks Shoog for sharing (and of course Joe Rasmussen!)
Cheers
Andrea |
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| Shoog |
Hi Andrea,
Good to see you gave it a go, looks good to me. The exact bias point needs to be carefully set to get the best out of this circuit- experimenting is the only way really.
If you try it on your main system please report your impressions so that others can decide whether to try it.
Just got back from a weeks holiday and my main amp is down bummer:bawling: .
Shoog |
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| maikel |
| quote: | Originally posted by Shoog
The power supply I used was a simple 5 stage CRCRCRC filter. IT is said in the original design that the superreg is central to the sound of the circuit.I cannot comment as I have never heard the superreg.
However in my small experience the simple RC filters often sound superior to regulated supplies.
The 156V supply can be varied but you will have to adjust the circuit to compensate. This shouldn't be to difficult.
Shoog |
Hi Shoog,
Did you manage to resusitate you're amp??
Can you post the 5 stage CRCRCRC filter schema?
Did you manage to compare the sound of the FVP5 to you're design??
I never build a tube-amp, and I heard that building a phono-amp is even more difficult because of sensitivity. But I want to build a real high-end phpno-amp (MC)
That's why I think it's more realistic to build you're amp than the FVP5.
What do you think, and do you have any tips / suggentions for me?
Thanx very much for you're comments and time
Maikel |
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| morricab |
well I haven't heard a Vacuumstate product without a superreg (Allen uses them in everything tube that he makes) but I can say with them his designs are among the best sounding I have heard personally. I have had a FVP at my place for about 1 1/2 months in the past and currently have an RTP-3D at my place for review. Both are seriously good preamps (the RTP-3D is probably one of the best). BTW, the FVP has now been replaced by his new SVP (six valve preamp). He is also using this superreg in his DPA300B amplifier (on the input stage).
If I were to build a new DIY design now I would seriously consider using this shunt regulator as it seems easy to implement and apparently from the sound of Allen's gear, it really works well. |
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| Shoog |
I never heard the FVP5 or a superreg. I would guess however from the reviews of the FVP5 that I have read, that the sound of my implementation is very similar. Mine is clean and clear with plenty of punchy but controlled bass.
I built a CCS loaded 5687 preamp and though I liked it a lot, it was far less neutral than my suggested design.
Mine is relatively simple to build and only used 3 valves plus a rectifier..
On the matter of the Power supply, I would suggest starting from the transformer you can lay your hands on and then using Duncanamps PSU2 simulation program to derive a suitable supply. I have used salvaged computer power supply caps with a final high quality bypass. The total current draw of the amp is about 16ma.
Good luck.
Shoog |
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| rdf |
| Hi Shoog, in testing I found that a green LED bypassed with a cap greater than about 25uF bettered the performance of red and IR LEDs, bypassed or not. Residual distortion was below the limits of a 24-bit DAC. It's a good solution for those who don't mind the cathode cap. Have you considered an IXYS chip for the current source? Though it's also probably a better technical performer the main benefit is simplicity. A couple of resitors and an IC. Might save you some work. |
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| Andypairo |
| quote: | Originally posted by rdf
Have you considered an IXYS chip for the current source? Though it's also probably a better technical performer the main benefit is simplicity. A couple of resitors and an IC. Might save you some work. |
Hi,
I was also thinking about it: if I were to use 2 triodes for the SLCF I would use the second triode for the bootstrap element and use a SS CCS to load the cathode follower.
This is because one of the schematics @vacuumstate suggests that there might be problems with the non-linearity of the Fet capacitance.
On the other hand, as John Broskie suggested while presenting the Aikido, loading a tube with a similar device is a good thing... so probably the all-tube solution is the best soundwise...
Cheers
Andrea |
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| rdf |
| My apologies for the at-work shorthand. I should have specified the circuit used to current bias the green cathode LED. The LED has a dynamic AC impedance of ~12 ohms, the 330 ohms looks close to a current source. A SS CCS however would afford a huge reduction in potential noise injection from the AC line and do a much better job injecting a truly constant current. |
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| Shoog |
I read somewhere that the FVP5 sounds better with the tube constant voltage element, however this requires another valve and a higher voltage. As I said originally this design evolved out of a split rail valve buffer, and introducing another valve at the top means that the rails would have to be unbalanced and precisely tuned. With the design as is everything fell into place with regards to voltages in a split rail valve buffer. When it evolved into its current form, again the voltages fitted nicely so I left things as they were.
I know that the triode based CCS is not a great performer, but is its performance good enough for the job. Are the swings of introducing a silicone CCS going to be outwayed by the sonic roundabouts of introducing silicone. Would we be chasing diminishing returns. Personally I think it performs well enough. With regard to noise issues, I run a very sensitive setup and I have never had any kind of noise issues from my preamp. If the rails are smooth enough then noise shouldn't be an issue. Again it works adequately- so why improve it.
The FET functions as a cathode/source follower which is the least prone to gate capacitance. How would you tell if it was a significant issue, there is no high frequency roll off to speak of. Use a better FET if you think it would help.
This is a carefully tuned design, once you start changing one element you have to retune all the other elements to compensate. If you want the performance of the FVP5, then its not significantly more complex than my design. I like my design because it performs well and has an elegant simplicity. Don't take offence, but I think the choices I made were the right ones. The original FVP5 is probably degrees more tuned, the design is out there so make your choice.
Shoog |
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| Andypairo |
Shoog you are right,
it's all a matter of choices :D
My post wasn't meant as criticism but as a sort of white paper (or loud thinking :clown: ) to brew my own version or to let someone else do something similar.
Of course a tube bootstrap would require more B+, especially if one wants to keep the tube CCS.
About your circuit, at which current does the SLCF work ?
I'd say about 10ma, can you confirm this?
Cheers
Andrea |
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| Shoog |
Nearer to 7ma through the SLCF.
If tweaking the design and wanting to include a valve at the top, it shouldn't be to difficult. You will need to think of around another 80-90V on the +B, you will want to keep the current through the voltage gain stage about the same, and also keep the voltage on the plate of the first tube about the same. I think the current through the first valve is about 4ma. As I suggested before using a pot as the last RC filter element of the first stage, will allow you to find the sweet spot by trial and error, and then substitute with a fixed valve resistor.
Shoog |
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| Shoog |
I tried running the first valve at higher currents, but for some unknown reason it sounded harsh and congested.
Shoog |
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| maikel |
Hi Shoog,
another stupid question :xeye:
Does you're schema include RIAA correction ?
I want to use a 'choke-input - voltage regulator - choke output' PSU. I included the schema as an attachment (as you can see it is the toccata psu designed by Loesch).
I think the zener D1N750 (4,7V) is for protection-purpose...
Thanx again,
Maikel |
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| Shoog |
"Does you're schema include RIAA correction ?"
Unfortunately not. I have had great success with Leosch's El Cheapo phono stage. Unfortunately it uses a higher +B than my circuit.
I don't see why the suggested PS wouldn't work excellently. I used a simple CRCRCRCR type power supply and it worked well. Can't say whether Leosch would sound better, though it probably will.
Shoog |
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| maikel |
| quote: | Originally posted by Shoog
"Does you're schema include RIAA correction ?"
Unfortunately not. I have had great success with Leosch's El Cheapo phono stage. Unfortunately it uses a higher +B than my circuit.
I don't see why the suggested PS wouldn't work excellently. I used a simple CRCRCRCR type power supply and it worked well. Can't say whether Leosch would sound better, though it probably will.
Shoog |
Hi Shoog,
Wow that's fast
The reason I searched for another PSU is that I read that the RCRCRCRCR PSU-design has quite a lot of voltage-drop and dissipation.
Furthermore I really think you're design deserves a very good PSU-design. Because I am looking for a really good (neutral and fast) sounding phono-design, I hope to combine best of you're and Loesch's world.
I have to tell you that I base my choices on what I read on different fora on the internet (thanx to google).
I have a basic knowledge of electronics. And as much as no practical-experience...
I have to start somewhere
Anyway, I really appreciate you're effort and suggestions.
PS: I have to find a way to recalculate the output voltage of this PSU design. |
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| Shoog |
Within reason an overvoltage PS can be trimmed down to any voltage simply by insertion of a series resistor in the positive leg. You have to know the current draw of the design to do this accurately (about 20mA for my design), but using a power potentiometer allows you to trim a variable amount. You can then substitute a fixed resistor of the appropriate value.
This works because the preamp draws a constant current.
If you started out with a 280-300V supply you could tap off at 250V for EL CHEAPO, and then add another stage of RC filtering in order to get down to the 165V(I think thats the right voltage) you need for my preamp.
I find that a practical approach is often the best when you aren't to certain of the underlying theory.
Shoog |
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| Shoog |
It appears that the suggested PS gives an output of 200V (set by the 200V zener). Lowering the AC input to about 200VAC and changing the zener would be all the modifications neccisary.
Shoog |
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| rick57 |
Hi Shoog
You probably know the price of a FVP5 kit is $3500 :bigeyes: :cannotbe: (per www.tnt-audio.com/ampli/vse_fvp5a_e.html - did they slip a n extra digit in?
What was the approx all-up cost of yours?
Thanks |
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| Shoog |
Mostly salvaged parts (old PC caps, back to back transformers) a few salvaged bases, salvaged alps stepped attenuators, salvaged LEDs - cost next to nothing.
PCC88's about €40.00 valve.
All in all about €100.00. A bit of a bargin in comparison.
Shoog |
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| rick57 |
Hi Shoog
salvaged = :up:
any ballpark idea on what it may cost with new bits :confused:
Around E 200, or E 400?
Cheers |
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| Shoog |
Depending on what you spend on a case really. Assuming you spend next to nothing on the case, expect to spend about €200.00.
I used an old Pioneer amplifier case for mine. All the valves are inside and I didn't have to buy any phono or power sockets. I would guess that would probably have saved me about €50.00
I personally don't much like having the valves out on show as they are more vunerable and also it makes it much harder to accommodate the finished unit.
I would point out that mine hasn't got a phono amplifier section or the superregs. Also mine only uses 3 PCC88's where as the original uses 5. Compared to many other high end kit out there I would say that the original was still good value for money.
Going back to the power supply. I tried using a bipolar transistor capacitance multiplier in my power supply. It definately sounded worse than a simple CRCRCRC power supply. I cannot comment on the power supply asked about before, as it uses FETs. Still it may not sound better than a simpler version.
Shoog
Shoog |
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| rick57 |
Hi Shoog
Sounds like a fine example of the Pareto rule at work ;)
It’s definitely made it onto the shortlist :up:
Cheers |
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| Shoog |
I never heard of the Pareto principle before, but it perfectly sums up my philosphy to most things. I think I am probably sacrificing about 20% of the performance of any given project but saving 80% of the cost. To me that is a fair trade. I do have a stock of quality components as well and they get used in critical places. It bores me rigid when I see a thread springing up on caps and resistors.
Shoog |
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| av-trouvaille |
Hello Shoog, others,
Thanks for sharing the results of your quest.
Some years ago I’ve build the FVP5 (rev. 1 of 30-11-98).
The result is more than satisfying in it’s original design.
In the meantime the shunt super regulation is ready and can be added.
There are some differences between your schematic and mine.
- In my version the lower tube of the second stage is a mosfet –ccs. The power supply is 200 volts, while in your case it is 156 volts. Did you deliberately settle on 156 volts?
- The plate resistor of the first tube is 25k (yours 19k). This might be related to the lower supply voltage?
- I wonder why R8 in your schematic (it is not in mine) is so critical in value (you mention 1k and 660 ohm, and the need to tweak this value). How can this be of influence on the sounds anyway. Can someone explain this? Is this related to interaction between the two stages?
- the resistor R4 in your schematic is 47k, while my schematic says 470k.
- the value of R5 in your schematic is split up in mine in 1m5 (with 0.22 uF in parallel) and 47k
- Can you tell what is the voltage at the cathode of the first stage with supplementary current circuit?
- I would like to learn if the supplementary current to bias up the LED into a more linear range in the first stage could also be applied to the led in the second stage in your schematic and if it would benefit the sound. Did you consider this?
Best regards, Arjen. |
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| Shoog |
Lots of good questions.
"- In my version the lower tube of the second stage is a mosfet -ccs. The power supply is 200 volts, while in your case it is 156 volts. Did you deliberately settle on 156 volts?"
A mosfet CCS will probably perform better, but might not sound better. I said this before that the triode CCS seems to work well enough. My original inspiration came from the Gainclone buffer circuit which Vacuum State use, on this there is a bipolar transistor on the top and a triode on the bottom. The designer must have made the decision to stack them in this order for a good reason. Also to substitute a mosfet CCS would require re-jigging all other components.
The 156V was the power supply I had and which I designed the rest of the circuit around. Other voltages would work but would need component adjustments.
"- The plate resistor of the first tube is 25k (yours 19k). This might be related to the lower supply voltage? "
The choice of the first plate resistor was made to place the plate at about 100V. This is a critical point as it sets the bias point of the second stage. I found through experimentation that this gave a nice balance between an overly full sound or an overly thin sound.
"- I wonder why R8 in your schematic (it is not in mine) is so critical in value (you mention 1k and 660 ohm, and the need to tweak this value). How can this be of influence on the sounds anyway. Can someone explain this? Is this related to interaction between the two stages?"
This follows on from my previous answer. It is a fine tuning mechanism to set the bias point of the second stage and hence the tonal characture of the whole preamp. This is why I strongly suggest using a 1K5 variable resistor so that you can adjust the tone of the circuit to your own taste.
"- the resistor R4 in your schematic is 47k, while my schematic says 470k."
Because the Mosfet only needs a small amount of voltage to operate compared to a valve, the 47K sets a lower voltage drop across the Mosfet.
The choice of values of R4 and R5 are relatively arbitary, its is the ratio that is important.
" Can you tell what is the voltage at the cathode of the first stage with supplementary current circuit? "
About 2.1V. This voltage will be relatively static with different currents. Without the supplemantart bias the sound is muddy. I did not consider it neccissary to apply supplementary current to the Red LED because it is passing about 7mA and is hence already in its linear range. I also think that because the bottom triode is somewhat isolated from the signal it is less critical. Adding supplementary current may bring improvements, I did not feel it was warrented.
I would say that it may be a good idea to substitute a lower gain triode for the voltage gain stage. This is because the gain is excessive and will probably overload the second stage. All that would be required would be to choose a low gain triode with a good operating point that places the plate at 100V. This may also bring sonic benefits as the ECC88 is not the best sounding triode and will add its sonic signiture to the preamp in a way that the SLCF not.
I did not design my preamp to better the original. I did it as an intellectual exercise. It may or may not come close to the original - I cannot say. All I can say is that it is very neutral in itself.
I would be very interested in what benefits are derived from adding the superreg.
Shoog |
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| av-trouvaille |
Hello Shoog, others,
Thanks for your clear reply.
This would be an interesting experiment.
* I would say that it may be a good idea to substitute a lower gain triode for the voltage gain stage. This is because the gain is excessive and will probably overload the second stage. All that would be required would be to choose a low gain triode with a good operating point that places the plate at 100V.
Yes, other tubes might be better. I did not try other tubes yet.
Does anyone have a suggestion for a tube that would be a drop-in replacement so one does not need to recalculate all values?
* This may also bring sonic benefits as the ECC88 is not the best sounding triode and will add its sonic signiture to the preamp in a way that the SLCF not.
First I'll try to add the shuntreg in the next months. That already is quite time consuming.
Best regards, Arjen. |
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| ErikdeBest |
Hi Shoog
I am building this crossover by John Broskie http://www.glass-ware.com/tubecircuits/18%20db.gif
Instead of the ECC88 cathode followers I am using a PCF80/ECF80 (I have a lot - who doesn't) with the triode as buffer and the pentode as CCS. The supply is a bipolar one, with rails at +100V/-100V. I am building it in a quiet modular version, that makes it easy to test other buffer circuits.
Now I see your FVP5 version and I thought of using it as the buffer, see how it sounds. As I already have the bipolar supply and want to keep the grid at ground potential, I copy pasted your schematic in paint and redraw it somewhat to adapt to my situation. What do you think, shall it work? Is it worth the trouble to build it? Other question, as I am like you, always trying to salvage parts (and I am noob on mosfets). I have some of these mosfets MOS-N-FET BS107 200V, 0.12A, 0.5W http://easymeasure.nl/bs107.pdf Can they be used instead of the IRF510 (first impression is that the rating are ok, N channel as the IRF, but the bs107 is a depletion one, whatever that means, mosfet).
Many thanks for the attention
Erik |
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| Shoog |
I designed and built a buffer circuit for a Gainclone. If you search the forum under my name and buffer you should pull it up. It uses split rails of +/-80V and has all the components worked out.
I cannot honestly answer your question on the substitution of mosfets. The N type is fine, but I don't know about the depletion mode.
Shoog |
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| ErikdeBest |
Hi Shoog
Many thanks for the reply. I searched in the chipamp forum, but could not find the thread.
The main reason why I ask if your version of the FVP5 could be adapted is your comment that this circuit does cut the feedback, but still provides a low output impedance. (I can't find it right now, maybe I dreamed about it). Not that I am againt feedback, but for me it would not be hard to just give it a try, see what happens and how it sounds. Although not hard to implement, it can take some time :D, but I will let you know about the results
Erik |
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| Shoog |
The circuit as you showed it should work without modification even with higher voltages. The only thing that might need adjustment would be the 47K resistor. Just don't forget to add a resistor to ground on the input.
If you do any comparisons I'am certain that there would be interest in your results.
Shoog |
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| ErikdeBest |
Thanks Shoog
I ordered 20 noval sockets, so experimentation can begin...well, still have to find a source for the mosfets.
Erik |
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| Andypairo |
| quote: | Originally posted by ErikdeBest
Thanks Shoog
I ordered 20 noval sockets, so experimentation can begin...well, still have to find a source for the mosfets.
Erik |
Try these http://cgi.ebay.de/10-x-IRFD220-Pow...1QQcmdZViewItem
not the same as you thought but dirt cheap and more than suitable for the job
Cheers
Andrea |
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| Andypairo |
| quote: | Originally posted by av-trouvaille
Hello Shoog, others,
Thanks for sharing the results of your quest.
Some years ago I’ve build the FVP5 (rev. 1 of 30-11-98).
The result is more than satisfying in it’s original design.
In the meantime the shunt super regulation is ready and can be added.
There are some differences between your schematic and mine.
- the value of R5 in your schematic is split up in mine in 1m5 (with 0.22 uF in parallel) and 47k
Best regards, Arjen. |
| quote: | Originally posted by av-trouvaille
Yes, other tubes might be better. I did not try other tubes yet.
Does anyone have a suggestion for a tube that would be a drop-in replacement so one does not need to recalculate all values?
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Hi Arjen,
which circuit did you use for the shunt super-regulator?
As for the questions,
R5 is split to isolate the cathode of the tube from the non-linear capacitance of the Mosfet, since the 0u22 shunts the larger resistor for AC signals
Using a different tube needs some calculations to be done, but the operating point can be quite easily deducted from the curves of the tube.
Pin-to-pin-compatible tubes with lower mu than E88CC (that I know of ;) ) are 6H30, 6N6, 6GU7, all in the ballpark of mu close to 20.
Gain is probably too high even with these.
I'm seroiusly thinking to use the SLCF as unity gain preamp, maybe after a trafo (that might add some boost if needed) .....
Cheers
Andrea |
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| Shoog |
Last night I changed the R8 to a 820R resistor. This seems a good compromise between the overbloom of 1K and the slight hissness of 660R.
I repeat - try a pot to set this to your own taste.
I'am now toying with the idea of doing an ultimate version. Up the voltage to 250V. Substitute a 12B4. Replace the top Mosfet with another PCC88. Add reed relays for switching. Might be good.
Shoog |
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| ErikdeBest |
Hi Shoog
Sounds interesting, that ultimate version. I am curious
Just one more question, also to andypairo!!
What is the dissipation for the IRF in this design? Lets take the original one...I assume it is about half the rail voltage (~80VDC) multiplied with the current through the tube (and that is...10mA?). that would make for a dissipation of about 0,8W. The BS107 (costs 6 cents, and is easy to get) would be to small, as its maximum dissipation is 350mW...even at 5mA of current ot would still be too small. The IRFD220 is rated at 1W, that would be brely sufficient for 80V/10mA...and probably I will be using 100V of rail supply. I will contact the guy from who I always buy and see what he has got in his assortiment.
Many thanks for reading
Erik |
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| Shoog |
The FET drops little more than 4-5V (can't quite remember off the top of my head), so with a current of 10ma you only need a dissapation of 5x0.010=0.05W. This is for quietsent conditions.
Shoog |
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| ErikdeBest |
Hi Shoog
Thanks, many thanks... And of course I was wrong. Let me, at least, try to explain...there must always be a drop across the plate, with the use of a resistor that is half the B+. WIthout thinking much I applied this logic to your circuit...but off course this is wrong, with an active load this drop can be quiet small...
The BS107 is rated 350mW, 7X the 50mW quiescent current. I think I will order some...
Erik |
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| kenev |
Just a dummy question: what does "bootstrap" mean? Or, when we say that, in that particular circuit, that the tube's anode is bootstrapped, what does this mean?
What modifications are needed if I want to replace the MOSFET bootstrap element with a tube?
Thanks,
Evangelos |
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| Shoog |
The function of the Mosfet is to test the output signal, invert it and apply the opposite voltage to the plate of the output valve. In this way the voltage across the output tube is kept constant in relation to the fixed current supplied by the CCS. Hence it is a Constant Voltage Constant Current buffer. This makes the output tube behave in a more linear way hence the name Super Linear Cathode Follower.
That is my understanding of the principle on which this circuit works.
If you want to know how to substitute a valve for the Mosfet, I suggested you visit the Vacuum State website and look at the original FVP5 circuit(s).
Shoog |
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| Shoog |
Important newsflash.*****
I just realised there is a mistake in the original schematic. The final filter stage of the schematic is shared in my preamp, whereas in the schematic it is seperated. This means that R8 is half of its required value. ie, 820R should be 1K6. Alternatively you can share the final stage between both channels.
Hope that saves some hassle.
Shoog |
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| Shoog |
I took note of peoples comments about there possably been a problem with the Mosfets gate capacitance. With the Mosfet we are using, this is about 100pf. This will effectively mean that the buffer will pass slightly more high frequencies than low.
I have done a little experiement to try to compensate. I placed a 100pf mica cap in parallel with the resistor to ground between the two stages. This should roll off the top end by a similar amount to the boost recieved by the gate capacitance. Hopefully this will flatten the response slightly. Can't say that I can hear much difference though.
Shoog |
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| Andypairo |
Hi Shoog and all,
I was thinking about a way to have moderate gain from this amp without changing tubes and it came to my mind that a fellow Italian diyer, Giaime, had made use of some feedback from the plate to lower gain in a E88cc based preamp.
His circuits can be found here:
http://giaime.altervista.org/plate_follower.html
What do you think about such approach?
Cheers
Andrea |
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| Shoog |
I don't see why it wouldn't work but I don't think its the best approach.
12B4 are very linear and extremely cheap compared to ECC88's.
This circuit doesn't need feedback. Infact the whole point is to eliminate feedback. So 12B4 = cheaper and better.
I put the circuit to the scope today. Putting a simple sine wave through it gave flat response all the way to about 50Khz, only after that does it start to roll off a tiny amount. There was no rise in response- which I half expected to find. It just goes to show that BBC broadcast quality is sibilant and not very good quality.
Shoog |
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| Shoog |
Hi all,
Had call to pop the hood of my preamp, so I though I would check the exact voltage on the plate of the first stage.
79.5V and 80.0V
So that the sort of figure you will be looking for. Sorry for my vagueness before.
Shoog |
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| Andypairo |
| quote: | Originally posted by Shoog
I don't see why it wouldn't work but I don't think its the best approach.
12B4 are very linear and extremely cheap compared to ECC88's.
This circuit doesn't need feedback. Infact the whole point is to eliminate feedback. So 12B4 = cheaper and better.
I put the circuit to the scope today. Putting a simple sine wave through it gave flat response all the way to about 50Khz, only after that does it start to roll off a tiny amount. There was no rise in response- which I half expected to find. It just goes to show that BBC broadcast quality is sibilant and not very good quality.
Shoog |
Where do you source the 12B4? (and at which price?)
Cheers
Andrea |
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| Shoog |
Ebay, ebay ebay!!!.
From what I understand they are a very underrated valve that goes for peanuts.
Shoog |
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| pchw |
Bring up this old thread just to give credit to what it is due.
I built this FVP5 with the help of Shoog, of course. I used what I had in my parts bin as much as possible with the exception of the irf510a, no exotic parts. Chassis and PT are from an Eico tuner. Refurnish the chassis and prep the chassis took longer than the wiring. The preamp started the first note about one month ago and had a very noticeable hum. To make a long story short, the hum is 99% gone, this after tons of advise from Shoog via email. A few experience to share:
= same as Shoog's, there is no regulation, simple 6x4 -> RCRCRCLC
filtering.
= I had DC heater supply but didn't help reducing hum and had noticeable improvement over AC, so they are gone.
= B+ is usually between 144 and 146, depending on the hours of the day.
= I have my first stage set at 84V vs Shoog's at 80.
= Both channels draw 28ma. First stage draws approx 5ma, 2nd stage
draws 9ma each channel. Seem to be a lot higher than Shoog's.
Now the most important part, how it sounds. One word, great. To put
it to perspective, I compared to a factory wired Transcendent Sound ground grid preamp and a p2p version that I built. Transparency/natural is the GG's long suit, but the FVP5 clone is just as good, but when it comes to imaging and vocal, it beats the GG hands down. The interesting part is that it wasn't obvious until I switched back to the GG for A/B comparison. The first I noticed when switched back to the GG was the fussiness of the 3D image. There seemed to be a layer of silk in between, kind of like, you can tell where is the singer and instruments but not laser sharp in focus.
In summary, the result is very positive. While I can't say how it compares to the real FVP5, it is well worth the money and the time to build it. It is simply a very good sounding preamp. I can see this to be my main preamp in the forseeable future until .........
Good listening!! |
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| Joe Rasmussen |
| quote: | Originally posted by pchw
Bring up this old thread just to give credit to what it is due.
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Hi Guys,
Thanks, it's nice to know one's work is appreciated. BUT as you guys know, the FVP was a complete Phono and Line Stage preamp. The Phono was designed by Allen Wright and the Line Stage by yours truly. Indeed I coined the term Super Linear Cathode Follower in the latter half of the 90's.
Re the constant voltage element that sits atop the CF tube, it is important to keep its input capacitance low as it can be quite non-linear. This means that if a Fet is chosen - choose high voltage that is equal to or greater than your rail HT and low capacitance. We originally used HexFets because they were easy to get and high voltage/rugged etc. - ended up choosing IRF710. But now I would tend to favour a bipolar device. I have used a Darlington pair made from TIP48 (or TIP48/50). I also have tried to use a single TIP50 and used the current derived from its Hfe. That way you can typically use a single 470K to 1M resistor to the HT (second R not required to form a voltage divider) and only a cap, say 0.47uF, to the output, cathode of the tube.
So there are a variety of options, and the parts are not exotic and yet work very well.
You can of course do what the new SVP does, and make the constant voltage follower another tube still. This requires that HT needs to be high enough.
Joe R. |
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| pchw |
Hi Joe,
I owed you a from the bottom of my heart, "thank you" especially my clone is only an one input line stage, no phono :-) I really like how it sounds and am trying to tweak it one parts at a time. Your comments came in the best timing. In fact, I am in the process of building an outboard power supply for the next iteration. While it is not super reg, it will be regulated supply all around. I will keep the current version around so that I can compare.
Thanks again!! |
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| Shoog |
Hi Joe,
Thanks for the input. Your ideas are much appreciated.
Since pchw copied my copy I have made a few subtle changes. I replaced the LED's in the buffer stage with simple resistors. After readjusting the operating point of the first stage I got a subtle but significant tightening of the bass. The sound is slightly less relaxed overall - all changes have to be carefully judged.
If I was doing it again I would drop the current boosting network in favour of a simple resistor from the HV rail. I don't think it would have any sonic impact, but it would be simpler to implement.
My next version will have a EF86 front end running at 1mA. I will take the screen from the output node of the buffer stage and use a voltage divider to DC bias the grid of the buffer stage off of the first stage anode. This should function as a sort of UL stage with the anode of the EF86 held about 80V above the screen, such that the screen will draw tiny current. It should also help to moderate the gain. I will also put another triode on in place of the MOSFET. I'm hoping this will all produce a significant step up. The only thing I haven't decided on is how to further stiffen the Constant Current Sink, and if there is much sonic gain in the effort.
| quote: | | While it is not super reg, it will be regulated supply all around. I will keep the current version around so that I can compare |
Be careful, my experience is that anything but the best regulators hurt the sound.
Good luck.
Shoog |
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| Joe Rasmussen |
Hi Shoog
Looking at what you have done, I am inclined to use the LED as a voltage reference with, say 1mA, on the grid of V1B - the CS. You have to draw that from a + source via a suitable resistor to give you 1mA. It could be the filament supply or, if you have to, from the HT. Then use a resistor on the cathode of CS (V1B) to adjust current.
BUT, to get the tightest CS, use a small suitable Fet instead of the resistor, adjust as a current source. Put a 1K resistor on the gate and then 100R (adjust) on the Source to give you the desired current. I am not sure IF I am painting a clear word picture here?
Re the the voltage gain stage, I am aware that it has a lot of gain for a Line stage. But I cannot stress the simplest and yet utmost importance of the "unloaded Triode".
Many look at the SLCF and make wonderful remarks about it. Yes, it is what it is! That is why it makes such a good UG (Unity Gain) buffer. BUT as far as gain is concerned, the key to understanding the sonic beauty of a Triode gain stage is: it sees virtually no load on the anode because of the high input Z of the SLCF buffer. I am not sure, but I recall another notable designer for Audio Note who understood this. In effect, when a Triode is unburdened of any load, it is effectively in a similar mode to that of constant current. I know this takes a bit of left field thinking, but it is fairly true as a Triode stage dealing with line level signals and very high voltage overheads. So as the Triode swings, there is very little/minimal change in current. And this is when they sound best. After that, the SLCF is good enough to hear that real Triode quality.
Re EF86, I am yet to be convinced. The problem remains, the high gain?
Joe R. |
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| Shoog |
| quote: | | Re EF86, I am yet to be convinced. The problem remains, the high gain? |
Since the screen constitutes 100% feedback, the gain should be as if it was triode strapped, and further reduced by the voltage divider to the buffer grids. I know what you mean about the triode probably been the best option overall - but I want to try this anyway. If it doesn't work out it should be a simple matter of triode strapping the EF86, which I understand to make an excellent triode in itself.
I will probably put a simple little FET in the CS cathode as you suggest.
At the moment I am planning a power amp build so this project will have to go on the back burner.
Shoog |
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| Joe Rasmussen |
| quote: | Originally posted by Shoog
If it doesn't work out it should be a simple matter of triode strapping the EF86, which I understand to make an excellent triode in itself.
Shoog |
There is a way to lower the gain stage of a Triode stage. Increase the local feedback by increasing the value of the cathode resistor. Of course, then you lose the current through the tube and up goes your anode voltage and you've lost your operation point. But what if you now make up the missing current by inserting a small parallel fet across the increased value cathode resistor. The anode voltage will return to normal, but the overall gain is reduced. This trick only works in this situation - it is not recommended as a cure-all. It just happens to work in this case where the anode sees effectively no load.
Joe R. |
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| Shoog |
Joe,
Attached is a picture of my idea for the next build.
I tried it with a 6AU6 and the screen and plate at the same voltage, it sounded grainy and the top end suffered. Smoking Amps then pointed out my error (to much screen current), so i rejigged it to the new scheme. The screen current should be well below 1mA most of the time so the buffer shouldn't brake a sweat supplying it.
I have no illusion that it will be an improvement - but it just might be !!
Shoog |
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| Joe Rasmussen |
Hi Shoog
OK, hope you don't mind if I critique your proposed schematic.
The voltage divider formed by R12 & R11 is a two-edged sword. The good: It will lower the noise floor. The bad: It will load the anode of the gain stage - bad for sound. I know which choice is more palatable - some noise and put the voltage divider on the input. One tube that could be tried for the gain stage is the U7/ECC82. Do a Manley trick and even parallel them if you want a single tube per channel (but I know some purists who don't like doing it - I'm pragmatic as well as purist). That will give you lower gain - have not tried it. But I did a U7 input stage on a power amp using similar conditions and it sounded very nice.
I do repeat: The unseen key to the excellence of the FVP5 line stage is not just the SLCF, but also the anode of the Triode gain stage sees a near infinite high impedance. In your instance R12 & R11 would need to be extreme high values - not practicable.
I violated your copyright and amended the schematic. The added Fet allows the parallel resistor to be much higher value and hence lower gain. With ECC88 there should just enough voltage for it to work.
If using EF86 and adding R2, you have introduced negative (loop) feedback in to a "Zero-Feedback" design. Sorry, but not impressed. If you are thinking 'Ultra-Linear" - then I have to admit NOT to be a fan. The best Triode (or Pentode) is where the input grid is totally in charge - does not happen with any form U-L. In transformer (power) outputs it is even worse if Push-Pull amps - now you have some real symmetry problems (think smearing). No, we are no U-L fans! Seriously thinking about starting a new trend... the anti-fan club. :smash: :smash: :smash:
My other suggestion - see my violated (your) schematic - in the SLCF I would use the LED as shown. You can even use TWO Leds. Feel like three? In theory this is a good thing as the resistor setting the current will be higher value. Of course a Fet current source here would convert that resistor to, like, huge equivalent R!
But, by all means build what you have in mind - but it takes away some of the things that makes the FVP5 so simple and yet so good.
Joe R. |
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| Shoog |
Joe,
I didn't quite understand the idea with the LED to stiffen the CCS - brilliant !!! Thats definately a keeper and will go into the next build. A string of 3 Green LED's would stiffen things up very nicely. It should also cut overall gain usefully. Is it concievable that this configuration could introduce a bit of PS cancellation ala the Aikido.
I appreciate where your coming from with the UL setup and i'm inclined to think your right. I have a particular fondness for the 5687 and this would make a good front end with a slightly lower gain, however it would run at a wasteful current of 10mA or so. Do you know of any other triode with moderate gain that sounds good at 4mA or there abouts.
Shoog |
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| skip_scratch |
| whats the gain of this preamp? |
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| Joe Rasmussen |
| quote: | Originally posted by Shoog Joe,
I didn't quite understand the idea with the LED to stiffen the CCS - brilliant !!! Thats definitely a keeper and will go into the next build. A string of 3 Green LED's...
I appreciate where your coming from with the UL setup and i'm inclined to think your right. I have a particular fondness for the 5687 and this would make a good front end with a slightly lower gain, however it would run at a wasteful current of 10mA or so. Do you know of any other triode with moderate gain that sounds good at 4mA or there abouts.
Shoog |
Ahah! You get the idea now re the LEDs.
I think there are plenty of triodes that sound good in this configuration, which is best will be subject to taste. But I admit that I have mostly been using 6922 in FVPs - but in power amp front-ends where the load has been typically 2M2 (not near infinite Z as in the FVP but still very high) I have used a variety of triodes and they have all sounded pretty good, which leads me to think that the configuration is more important than the choice of triode.
The following does have an advantage, use a triode that gives you a higher voltage across the cathode resistor for the Fet to work with. So ECC82/U7 has moderate gain and fits the bill. I have not tried it in an FVP but it did work a treat in a power amp front-end.
Looking at the curves of 5687 published www.diyparadise.com/tubeloadline/tubeloadlines.html, with a plate voltage around 150V and 5ma it indicates -9V, that should be great for the Fet. Make anode load 20-25K, try cathode resistor 2K2 - these should be starter values.
Divide anode/cathode values is around 10, so AC voltage gain cannot be higher than around 10 - it ought to be a little lower than that, rule of thumb. Try it and see. If you then want to further decrease gain, increase cathode resistor to 4K7 and then add Fet CS to take up the slack current - and get the same total current the same and the anode voltage will settle back to where it was with the 2K2 resistor.
Take the above values only as a guide, OK.
Joe R. |
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| Joe Rasmussen |
| quote: | Originally posted by skip_scratch
whats the gain of this preamp?
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See the previous posting. What would be regarded as the desirable gain? Max 10, if lower - will somebody name it? Five?
Joe R. |
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| skip_scratch |
| i'm using a unity gain amp and lower sensitivty speakers so need lotsa gain in the preamp |
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| Joe Rasmussen |
| quote: | Originally posted by skip_scratch
i'm using a unity gain amp and lower sensitivty speakers so need lotsa gain in the preamp
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That is not a problem - it's only for those who think the FVP5 line stage has TOO much gain.
Joe R. |
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