There is always room for improvement. The 'Guanzo' I am using in the interim, however I got it to use as a buffer between SS pre to amp, not as a preamp but it does surprisingly well as a preamp in the meantime. I have not made the JP200 yet, I have the parts but was waiting to see if someone could come up with a practical modification that was not SS to reduce the Z-value without compromising the sonic quality.
...however now - the 6C45 after reading about it, is something I am quite interested in ... there is some pretty good comments made about it and the very low impedance sounds very good...
...however now - the 6C45 after reading about it, is something I am quite interested in ... there is some pretty good comments made about it and the very low impedance sounds very good...
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Hi Douglas
Its obvious the 12AX7 is not usually used for such an application normally - ie: low impedance Z-value; and is high gain, which is why some substitute 12AT7/12AU7 or similar with some minor mods - which is why I was asking for modification suggestions; however --- on reading about the 6C45 and its very low Z-value - I am now quite interested in building a preamp using them - so if you can supply a detailed schematic at some point in the near future - ie: of the one you made; it would be much appreciated.
Are there any other things you would like me to look up??
Can it go as low as 2ohms or even 10ohms?? 2ohms would be super...
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Douglas, no problems outside of the fact that you have set your own terms as to the transaction for which TS would then receive a schematic, seemingly outside of consultation with TS, and on a public forum.
The question you posed is indeed akin to 'asking the number 5 if it were pregnant', in that it is the question itself which does not make any sense, that sentence cannot be finished in any correct manner, and you know this.
Hanze.
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Reality can bite, whats your best buy price on 6C45 per item, and have you checked the sample variance as per the datasheet - as Douglas says, you will need to buy multiples and then find a 'matched' pair, which may or may not stay matched, especially in a push pull circuit when you want zero DC current (imbalance) through the core.
Oh, did I tell you I ran an ME850 for a couple years with various tube front ends?.. yeah, think I did. Twice.
Anyway.
Hanze.
Just a few general words of warning WRT 6C45
a/ They love to oscillate, take precautions
b/ They have a fairly large variance between tubes of gm etc. It's a good idea to get matched pairs.
To get Z down below 10 ohms you will either have to use some feedback or a step down OP transformer.
No free lunch on this one.
Z
a/ They love to oscillate, take precautions
b/ They have a fairly large variance between tubes of gm etc. It's a good idea to get matched pairs.
To get Z down below 10 ohms you will either have to use some feedback or a step down OP transformer.
No free lunch on this one.
Z
OK ... so Douglas an engineer stated he got his working quite well, so he must have addressed all those issues - I would imagine...
...its all starting to get ambiguous again. I need a solid schematic. I'm starting to wonder about using the JP200 but with the following mods as done by 'Norbert' ie:
- Use ECC81 / 12 AT7 for the cathode follower position (T3A and B) to lower output impedance
- Replace C4A and B with silver mica 8 pF / 500 V and place a silver mica 22 pF / 500 V capacitor between grid and cathode of T1A and B to fight self oscillation
Lowering gain from 20 to 12dB:
- Change R5A and B to 33 kOhm
- Change R3A and B to 1 kOhm
...there are other resistor/cap values he changed as well, but these appear to be the main ones...
ie: further changes...
- Replace all resistors with Takman REY 0.5W metal film types
- Change R7A and B to 1kOhm
- Change R10A and B to 1.5 kOhm
- Change R14A and B to 100 Ohm
- Add C9A an B (which are not supplied with the kit) two TAD Oil Caps 0.25 uF / 600 V
- Replace C1A and B with Ampohm Aluminium Foil, Paper in Oil Cap 0,047 uF / 630 V
- Replace C2A and B with Ampohm Aluminium Foil, Paper in Oil Cap 0.1 uF / 630 V
- Replace C5A and B with Ampohm Aluminium Foil, Paper in Oil Cap 1.0 uF / 630 V (depending on the imput inpedance of your poweramp you might need a bigger value)
(I am using Ansar supersounds for this one)
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Hi TS,
I haven't time to go through the schematic, but just a couple notes.
Paper in oil. Avoid those at all costs. Use something like a Polypropylene or a metallised Polypropylene capacitor instead.
No. Use good quality metal film resistors. For a "best" resistor, use Dale resistors which can be selected for temperature co-efficient. Be careful to go with the size that has enough voltage rating for the circuit. That's right, resistors can be limited by voltage drop before their power rating is approached. The voltage ratings increase with power dissipation ratings. If you look at the data sheets you will find another horrible truth. Select resistors with a low voltage co-efficient. That's right, resistance can change with the instantaneous voltage across them as well. Like there wasn't enough to worry about.
For your purposes, use Metal film resistors that have a high enough breakdown voltage rating. Don't sweat the rest for now.
-Chris
I haven't time to go through the schematic, but just a couple notes.
Paper in oil. Avoid those at all costs. Use something like a Polypropylene or a metallised Polypropylene capacitor instead.
Get real!!!
No. Use good quality metal film resistors. For a "best" resistor, use Dale resistors which can be selected for temperature co-efficient. Be careful to go with the size that has enough voltage rating for the circuit. That's right, resistors can be limited by voltage drop before their power rating is approached. The voltage ratings increase with power dissipation ratings. If you look at the data sheets you will find another horrible truth. Select resistors with a low voltage co-efficient. That's right, resistance can change with the instantaneous voltage across them as well. Like there wasn't enough to worry about.
For your purposes, use Metal film resistors that have a high enough breakdown voltage rating. Don't sweat the rest for now.
-Chris
I already have polypropelene capacitors with solid silver leads aka 'Ansar supersounds' for this project...
(why avoid 'paper in oil' just out of curiosity??)
Hi TS,
The only thing worse than PIO caps are paper sealed in wax.
They are too large and often not tightly wound to the extent of modern capacitors. The Q is lower for these (higher dissipation) and there is no fix for that save putting in a good capacitor.
Generally these types of parts come with a nifty story as to why they are better but represent poor value for the dollar even if they were not worse than normal industrial stock. Silver leads don't help one iota either. Audio jewelry, that's all that stuff is.
The best industrial capacitors can be awfully expensive - don't get me wrong, but they perform the best and can be measured to prove it. Listening tests also back up the readings. I hate to ask, but those caps with solid silver leads ... how much were they and what value did you get?
-Chris
The only thing worse than PIO caps are paper sealed in wax.
They are too large and often not tightly wound to the extent of modern capacitors. The Q is lower for these (higher dissipation) and there is no fix for that save putting in a good capacitor.
Generally these types of parts come with a nifty story as to why they are better but represent poor value for the dollar even if they were not worse than normal industrial stock. Silver leads don't help one iota either. Audio jewelry, that's all that stuff is.
The best industrial capacitors can be awfully expensive - don't get me wrong, but they perform the best and can be measured to prove it. Listening tests also back up the readings. I hate to ask, but those caps with solid silver leads ... how much were they and what value did you get?
-Chris
The input impedance was 68k ohms, I did not need such low drive impedance. You seem fixated on low drive impedance, you dont need to be. If I read correctly your input impedance is also 68k ohms, and really.. you could drive it with 6k8 and not loose (much) sleep. 150 ohm will do a very good job.. 10 ohms is just pie in the sky.
So to answer your question, I have to refute the premise of that question and report accurately with the facts.
The original (unmodified) design was to force the user into using their pre-amp. Did you not notice how simple it was to change the input impedance?.. there is a reason for this, it is called their 'plan B', it is built into the design and you did not have to replace any parts.
And yes, plan B works better for those who do not want, nor need to be confined to using their pre-amp. Plan A worked better for retailers who would demonstrate how the ME pre amp would really be the only suitable device to drive their amplifiers with. Did they sell many ME pre-amps?.. I bet they did.
If you want to break free of using the ME pre-amp, and it seems you do - you can change the input impedance to 68k, seems you did.. and forget this 10 ohm drive impedance, it no longer applies. Its a bit like going from black and white to a colour TV set.
Anyway, I've told you what I think, all the best with your project and the decisions you need to make.
Hanze.
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Incorrect. The original design grew out of a theory proposed by Peter Stein, that involved building a power amp whose input impedance more closely matched the impedance of the feedback network (on the other side of the diff amp). Otherwise identical amplifiers were shipped to listeners (I was one) and those listeners were asked to comment on any sonic differences. Obviously, listeners were required to be using a low output impedance preamp. The differences were quite audible. Hence, subsequent designs were built with a low input impedance.
Due to pressure from DEALERS, Peter incorporated a high input impedance option, sometime around 1992-3, so that poorly specc'd preamps could be used. Further and for the record: There are quite a large number of suitable preamps that will operate with the low input impedance ME power amps. Models from Krell, Mark Levinson, Classe', Boulder and many, many others will be entirely suitable. Also, be aware that the original premise for building a preamp with a low output impedance was to deal with several issues:
* The ability to drive very long interconnect cables.
* The ability to reject interference (particularly when mated with a low input impedance load).
* The ability to operate headphones direct.
Of course they did. ME preamps are something of a high end bargain. However, there are plenty of other choices from the like of Krell, Levinson and the others, should people not wish to use and ME and save a bundle.
He has already changed the input impedance.
Cheers Chris.. yep. I think TS design goals are not so challenging now that he has moved to 68k input impedance, he could use (within reason) any tube stage that he liked and not be concerned with output impedance (within reason).
Hanze.
Yep, we know that.. now he's free from ME designs pre amp as a requirement to drive his amplifier.
The rest of what you wrote is marketing BS.
TS himself said it sounded better with 68k Zin, as did my ME850.
You need to ask what you are contributing to TS's cause - not sure we need a lecture on ME marketing here.
Hanze.
Perhaps a presentation to the historical society of arcane design and market influence would better accommodate your position.
Here, you have no relevant position which might serve to assist the original posters ambition, which is how to best drive a power amp with input impedance 68,000 ohms. (the brand is irrelevant).
I would ask if you might agree with the above, yet have the feeling that it will just add to the pile of tripe.
So, would you agree?
Hanze.
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I have already provided my suggestions on how best to operate the ME550 (regardless of how the input impedance has been set). Since I have operated the ME550 with a huge variety of different preamps (valve and SS), I am in a position to know what works well and what doesn't. In fact, one of the best valve models I used was a Conrad Johnson Premier 16. No idea of the topology. Another excellent sounding preamp was the late Allen Wright's Realtime preamp. Schematics are readily available. Obviously, the higher input impedance should be selected for these preamps.
My suggestions were rejected by the OP, without the original poster bothering to take the time to listen to either my suggestions or the alternatives.
What I didn't and don't agree with, is your characterisation of why some of the ME power amps were supplied with a low input impedance. It was done for good, solid, engineering and sonic reasons, not financial ones.
My suggestions were rejected by the OP, without the original poster bothering to take the time to listen to either my suggestions or the alternatives.
What I didn't and don't agree with, is your characterisation of why some of the ME power amps were supplied with a low input impedance. It was done for good, solid, engineering and sonic reasons, not financial ones.
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