So many questions....
Bipolar Darlingtons sounds best when operating in a constant voltage mode, they sound amazing, musical and a certain grip, no vagueness. The key is to keep voltage swing away from it when used in critical parts of the circuit. Here the input is voltage and the output is the current, voltage swing kept to a minimum. It is in fact a transconductance stage.
To give credit due, Terry Demol also provided some help in device selection. But the key credit is to the late Rowan McCombe, who suggested the use of Darlingtons in the Sony SACD upgrades output buffer stage, which was a small solid-state version of the SLCF and it sounded better than anything we had tried before.
If I was to release this line stage schematic, it would not be public domain, it will be copyrighted but single license allowed for DIY use, in the same way as I do for the Elsinore speaker design.
Max voltage swing will not match the RTP, but do I expect it to match the RTP line stage sonically? Yes, I would hope so, or else I would not suggest it. I know that it is not tubes, but it will be incredibly less inexpensive to build. But you will see that the schematic has features that are identical to the tube RTP, but can be scaled down to using much lower voltage.
If I do post it, will this be the right thread, since this is under tubes and not solid state? I could post it here and if it garners interest, start another thread under solid-state?
Yes, a separate thread, that is what I was thinking too. I need a line stage and this is what I need to build. A few things on my plate right now, but winter is fast approaching, so that is a suitable winter project.
But below is a sneak preview with no values (they need to to worked out, but I have initial values in mind), there should be no difficulty in seeing the similarities of the RTP Line Stage. It appeals to me to make a preamp that only uses all Darlingtons and nothing else. All are NPN. It's my way of tipping my hat to Rowan as that was one of the last things he told Allen when he visited us down here, it must have been around 2001.
Any comments? Please keep in mind a new thread would be required. But I just wanted to see if you guys could spot the RTP similarities. The 24V supply may need to be a bit higher and many of the values in the circuit suggests themselves. Input current about 5mA per side (10mA in the long tail CS) and 10mA in each of the Buffers. That would make around 60mA in total for both channels. I have some special ideas for the power supply, it will use Solid-State Inductors valued around 100 Henries for the main rail, then a SuperReg to follow. But I have yet to fully sketch that out.
But below is a sneak preview with no values (they need to to worked out, but I have initial values in mind), there should be no difficulty in seeing the similarities of the RTP Line Stage. It appeals to me to make a preamp that only uses all Darlingtons and nothing else. All are NPN. It's my way of tipping my hat to Rowan as that was one of the last things he told Allen when he visited us down here, it must have been around 2001.
Any comments? Please keep in mind a new thread would be required. But I just wanted to see if you guys could spot the RTP similarities. The 24V supply may need to be a bit higher and many of the values in the circuit suggests themselves. Input current about 5mA per side (10mA in the long tail CS) and 10mA in each of the Buffers. That would make around 60mA in total for both channels. I have some special ideas for the power supply, it will use Solid-State Inductors valued around 100 Henries for the main rail, then a SuperReg to follow. But I have yet to fully sketch that out.
If you went the transformer route at the output, then I suspect the voltage would need to be much higher, and more voltage swing, to compensate for the step-down in voltage (which is also necessary to get a low output impedance). Yes, the down side is the cap and I am likely going to use 4.7uF min with a Miflex 0.22uF (yum) By-Pass cap. I already have these here (8 caps). And the solution is a lot less expensive than a transformer solution. The power supply also a lot cheaper as it avoids hundreds of volts.
If Allen had been around, I think he would have liked this approach. The cost is hugely less that an RTP Line Stage and that is a major attraction, and I already know how good the circuit is, just look at the power amp schematic I posted in #803 which I am listening to right now. I hate to say it, but this is the stereo gain stage of my current amplifier and I am sorry, but sonically it is superior to the gain stage used in any RTP preamp - and I have built more RTPs than any of you here. This is why I now use solid-state as a front-end and EL34 tubes on the output stage (which have much lower Anode impedance than Triode mode, "Beyond Triode"). We never shied away from from using solid-state and even the gain stage of the phono stage, there you will see a Fet in the RTP Phono Preamp. Mixing tubes and solid-sate? If it works, then we did it. The Fet solved several solutions and one was that it did not drift with age, you could match the two phases (Hfe current) and it would stay like that. The other was lower noise still.
This is my current RTP solid-state gain stage. Sorry, but it is better. I would love to still be using tubes. And I still do where they still can't be beat:
If Allen had been around, I think he would have liked this approach. The cost is hugely less that an RTP Line Stage and that is a major attraction, and I already know how good the circuit is, just look at the power amp schematic I posted in #803 which I am listening to right now. I hate to say it, but this is the stereo gain stage of my current amplifier and I am sorry, but sonically it is superior to the gain stage used in any RTP preamp - and I have built more RTPs than any of you here. This is why I now use solid-state as a front-end and EL34 tubes on the output stage (which have much lower Anode impedance than Triode mode, "Beyond Triode"). We never shied away from from using solid-state and even the gain stage of the phono stage, there you will see a Fet in the RTP Phono Preamp. Mixing tubes and solid-sate? If it works, then we did it. The Fet solved several solutions and one was that it did not drift with age, you could match the two phases (Hfe current) and it would stay like that. The other was lower noise still.
This is my current RTP solid-state gain stage. Sorry, but it is better. I would love to still be using tubes. And I still do where they still can't be beat:
Thanks Joe. The sand/glass combo I have seen occasionally was tubes for voltage gain and mosfets for the current/output stage. Seems like you have found reason to go the other way round.
In a way I am relieved to see this project emerging. The RTP3 has been on my bucket build list for a long time, but cost and complexity have always deterred me. There are not many fully symmetric amp circuits around, this will cover the preamp part…
In a way I am relieved to see this project emerging. The RTP3 has been on my bucket build list for a long time, but cost and complexity have always deterred me. There are not many fully symmetric amp circuits around, this will cover the preamp part…
Not sure if I responded to this. Ended up not using any resistor, even if on the surface it should put the first tranny into cs mode as you say. But, the whole cascode stage was required to give a gain of around 300x. And it was not a problem. But this was also driving two resistors (of quite high value) and effectively modulating a standing current of around 0.5mA. This puts the EL34 tube gain down under 2x (local feedback) and also makes the amplifier very quiet. It's hard to explain, but if we sat down together with a piece of paper we could scribble on, then it would be easier to explain. The result is the best amplifier I have heard, about 32 Watts per channel and clips like a triode (it does sound like a hundred Watter easily). Analyse the circuit and you will realise why the resistors nobles the particular job which is quite complex. Low voltage swings though, no problem.
@Joe Rasmussen: I am curious about the circuit. Is your intention to make PCB's / kits available once you feel the circuit is operating satisfactory ?
Oh I know that part 
, which was one of the reasons I asked this early in your project stage, sort of what the future might hold. I am technically capable of building the original one, just have not come around to it, same thing with the DPA-300B. I feel like exploring Susan Parker's source-follower transformer loaded amplifier atm
, which was one of the reasons I asked this early in your project stage, sort of what the future might hold. I am technically capable of building the original one, just have not come around to it, same thing with the DPA-300B. I feel like exploring Susan Parker's source-follower transformer loaded amplifier atmPurchased a ready built (used) Vacuum State RTP3D preamp two years but which needed a shunt regulator replaced. Did this with a vacuum state kit and commissioned three days ago and going through the familiarisation phase using phono input.
One of the things I am finding is that the sound really opens up after about 2 to 2.30 hours.
Is this normal for this design? Other tube designs usually get into stride after about an hour.
Also slightly noticeable hiss from one channel. Does this mean the phone tube need replacement?
One of the things I am finding is that the sound really opens up after about 2 to 2.30 hours.
Is this normal for this design? Other tube designs usually get into stride after about an hour.
Also slightly noticeable hiss from one channel. Does this mean the phone tube need replacement?
Taking on buying and sorting an RTP3D is a significant undertaking.
Noise on one channel? It could be a number of things, and a noisy tube is a possibility.
How long the preamp needs to be on, that is subjective. Maybe others might comment on that. Allen pretty much left it on all the time. I stayed with him a number of times over the years as well as the old Wentworth Avenue location. This place brings a few memories back:
Noise on one channel? It could be a number of things, and a noisy tube is a possibility.
How long the preamp needs to be on, that is subjective. Maybe others might comment on that. Allen pretty much left it on all the time. I stayed with him a number of times over the years as well as the old Wentworth Avenue location. This place brings a few memories back:
Hi Joe,
Thanks for getting back to me.
I purchased the RTP from US hoping that it will work from get go. But for some reason I was rewarded with one of the regulators blowing up on switching it on the first time. Hence sorting it out was a necessity!
Re noise, I will try changing the tube and checking the outcome.
Thanks for getting back to me.
I purchased the RTP from US hoping that it will work from get go. But for some reason I was rewarded with one of the regulators blowing up on switching it on the first time. Hence sorting it out was a necessity!
Re noise, I will try changing the tube and checking the outcome.
Congratulations on getting this great piece of kit! Your experience with warmup is consistent with mine - I normally switch the preamp on a couple of hours before I plan to do any serious listening. It certainly improves over this time, starting off sounding quite soggy, then going through a stage of "opening up". As Joe said, Allen always insisted that until it has been on for a few hours you are far from hearing it at its best. I did once intentionally leave mine switched on for a day, and it did seem to sound more expansive and dynamic, but I certainly wouldn't do that all the time, given that there are twelve values and a couple of highly biased SuperRegs in the box (and definitely not during the present heatwave in the UK!).
My feeling is that the hiss indicates an ageing valve. The trouble with this design (as well as its strong point) is that imposing constant voltage and current for the main active devices can hide local issues - I found this once with mine where a drooping regulator (not a VSE SuperReg, I should say) meant that the input valve was severely underbiased, but the preamp still sounded fine until it eventually dropped out.
Alex
Thanks for the detailed reply Alex
I must compliment you on the excellent web pages you had put up on your RTP diy journey. It was pivotal in my desire to own this preamp.
Another discovery over the week end is the need for vibration isolation, as has been my experience with all my other bits of kit. The improvement is always amazing.
Cheers!
I must compliment you on the excellent web pages you had put up on your RTP diy journey. It was pivotal in my desire to own this preamp.
Another discovery over the week end is the need for vibration isolation, as has been my experience with all my other bits of kit. The improvement is always amazing.
Cheers!
I'll second that. Alex - your pages inspired me to move forward with my own build - still in progress. I've got the power supply chassis completed, but have only barely started on the audio circuitry so far. I'm hoping to get it finished this year, but have a couple other projects I need to wrap up first.
I wonder if anyone have considered replacing the choke and Pi filter with ultracapacitor power supply. I am ofc thinking about what is going on over the Ian Canada "club" .. LOL. But on a serious note, the latest endeavour of ultra capacitor power supply got me thinking. If there is noticeable benefits on DACs, streamers/transports and master clocks, then surely it must benefit the pre amplifier.
A super capacitor supply is surely above choke and pi-filter and above battery's.
A super capacitor supply is surely above choke and pi-filter and above battery's.