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Solid state phase splitter
Solid state phase splitter
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Old 8th February 2019, 09:56 PM   #11
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by Osvaldo de Banfield View Post
This is undoubtedly true, but in such a case, then go for a full SS amplifier. Personal taste only: I dislike mixing SS and tubes.
Understand and agree about the personal taste part.

My personal taste goes the other way to yours. I have struggled for a long time to try and get good guitar tone with an entirely SS amplifier, and have never got anything better than just acceptable. So pure SS guitar amps don't thrill me.

In my opinion, it is exactly the differences between MOSFETs and valves that makes it so interesting to mix and match them:
  1. Valves are great at making distortion that sounds good with guitars; MOSFETs aren't.
  2. High-ra preamp valves (like a half-12AX7) are bad at driving any kind of difficult load; MOSFETs are really good at this, so they make excellent buffers for triodes.
  3. Triodes struggle to generate a voltage gain of 60 times (36 dB); MOSFETS are excellent at producing clean voltage gain for FX loop recovery, active tone controls, tremolo oscillators.
Because of these differences, to me, it makes a lot of sense to use valves as musical distortion-generators, and MOSFETs where we need audibly transparent buffers or perfectly clean voltage gain.


-Gnobuddy
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Old 9th February 2019, 01:27 AM   #12
Printer2 is offline Printer2  Canada
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Don't want to mention the name of the amp and manufacturer. Seems the schematics get unavailable on the sites that post them.
YouTube
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File Type: jpeg 79C227DC-F5A5-44D2-92D0-3FFEA41B276C.jpeg (29.3 KB, 83 views)
File Type: png E050EA14-9A40-4191-BE84-243E85FF3BEF.png (18.3 KB, 87 views)
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Old 9th February 2019, 06:20 PM   #13
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Printer2 View Post
Don't want to mention the name of the amp and manufacturer.
YouTube
Works fine if you want a PI with no audible distortion.

You could warm up the cleans elsewhere with real valves, but it sounds as if that particular amp designer didn't bother. I'm guessing the target demographic for that model doesn't care too much about clean tones, and is okay with the thin steely sound.

IMO, this is what good clean tone sounds like (start listening at 30 seconds into the clip if in a hurry): Fender '65 Twin Reverb Guitar Amp Clean Tone Product Demo - YouTube

I have yet to hear that sort of "valvey" or "tubey" cleans from any solid-state amp, though the Boss Katana 50 and 100 are getting pretty close. Not straight MOSFETS in analog circuits, though, but rather, hundreds of thousands of lines of computer code running on a DSP chip.


-Gnobuddy
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Old 10th February 2019, 02:16 AM   #14
Printer2 is offline Printer2  Canada
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I am pretty sure the PI of the twin is in the clean zone of its range of operation in the clip. I'm pretty familiar with the Twin sound as the first four years of my musical endeavors had my cousin playing through one. I was just showing the LTP circuit with Mosfets as it seems to provide gain, was not commenting on the sound as I have not tried it. If it remains as clean as the 100% NFB splitter as described on the previous page it might be an option to someone also.
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Old 11th February 2019, 01:44 AM   #15
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Printer2 View Post
If it remains as clean as the 100% NFB splitter as described on the previous page it might be an option to someone also.
Agreed, that thought has crossed my mind as well. It would work as a cathodyne substitute that does not suffer from "nipple distortion" like a real cathodyne or source-o-dyne.

The attached image shows the problem. The zener diode mimicks what happens when grid current starts to flow in an overdriven output valve.

In this example, I used a 10V zener, so if Vout1 exceeds 10 volts peak, the zener starts to draw current. This acts like bypassing the FET source resistor, which bumps up the voltage gain to the FET drain. This in turn shows up as "nipple distortion" on the second PI output, Vout2 (blue trace.)

The MOSFET long-tailed-pair you showed will not suffer from this problem.


-Gnobuddy
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Old 11th February 2019, 12:49 PM   #16
klingo is offline klingo
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A 47k/100k 1/4w resistor is as cheap as efficient to reduce the mA hungriness of the following power tube...MOSFET are nice in this application with the usual countermeasure...be happy and do not hit the rail ;-)
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Old 11th February 2019, 02:30 PM   #17
Metalgarri is offline Metalgarri  Netherlands
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Quote:
Originally Posted by Printer2 View Post
I am pretty sure the PI of the twin is in the clean zone of its range of operation in the clip. I'm pretty familiar with the Twin sound as the first four years of my musical endeavors had my cousin playing through one. I was just showing the LTP circuit with Mosfets as it seems to provide gain, was not commenting on the sound as I have not tried it. If it remains as clean as the 100% NFB splitter as described on the previous page it might be an option to someone also.
I see totally the point for Fender style clean amps. What do you think about the mosfet phase splitter that you are showing, for driving a power stage in an high gain metal amp? Do you think the phase splitter is also used to color the sound, or everything comes from the preamp in this style of amps?

Anyway, I own the amplifier that has that phase splitter (still not naming brands), and they say that "a well designed mosfet PI has the exact same performance as a tube PI, therefore there is no need to use an extra valve".

I'm tend to not trust exactly what they mainly because they call their amps "all tube", but they are as hybrid as any valvestate.

The approach I would like to use is to switch to solid state only where it doesn't really matter sonically, i.e. the rectifier is one, and I was wondering if it can be the same for the case of the PI in a heavy metal scenario...
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Old 11th February 2019, 07:03 PM   #18
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by klingo View Post
A 47k/100k 1/4w resistor is as cheap as efficient to reduce the mA hungriness of the following power tube...
I agree that large grid-stoppers are one of the keys to managing cathodyne distortion.

But as the attached image shows, there is still distortion even with a 47k grid stopper. I've used a typical Fender value of 56k for the cathodyne's cathode and anode resistors. When the output valve starts to flow grid current, that effectively puts the 47k grid stopper in parallel with the 56k cathode resistor.

That drops the effective cathode resistance to 25.55k, bumping up the voltage gain at the anode from nearly unity (1.0) to nearly 2.2 times.

Because the cathode resistor (or source resistor, in this case) is so large, IMO it is virtually impossible to make the output valve grid stopper so big that there is negligible "nipple distortion". If we make that grid stopper too big, there might be unwanted treble roll off within the guitar frequency band.

The long-tailed-pair circuit doesn't suffer from this problem, and if we build it with MOSFETs, it's also cheap and compact.


-Gnobuddy
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Old 12th February 2019, 12:28 AM   #19
klingo is offline klingo
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Yes you're right, it's better to lower the Rsource/Rstopper factor. the LND150 is supposed to stand 3 or 4 ma at a "wrong" bias voltage of 100v (here we design at 1/4 B+ around 80v), 33k R source&drain should be safe. with a 100k stopper it's hard to show the nipple.

If we assume that total input capacitance of an EL84 pentode mode is about 35pf including stray, the 100k stopper give you less than .2db attenuation at 8khz and the 220k+drain Zout will reach -1db at a frequency the guitar speaker & guitarist ear (the golden one, next to the splash cymbal) will ignore.

Now we should take care of the total grid load resistance which is 300k max/grid bias and 1M/self bias for the EL84 so 220k+100K is not that bad.

the drawback might be harder grid current clipping from a higher source impedance...should we consider using a 2x115v-2x24v small stepdown toroid load in a LTPI?

The long tail pair could also suffer from spike effect according to MerlinB, if anode/drain voltage of the inverting device is clamped due to grid current then it is more a cathode/source follower and pass all the signal to the non-inverting device instead of 1/2...or something like that. The stopper may be less than with the cathodyne , you have to sim or measure it to be sure.
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Old 12th February 2019, 12:48 AM   #20
Printer2 is offline Printer2  Canada
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Quote:
Originally Posted by Metalgarri View Post
I see totally the point for Fender style clean amps. <snip>
No idea on the circuit performance, I do not have the time I would like to play with electronics even though I have the parts to try the circuit. It is the playing/listening/comparing that takes all the time. Hopefully not too many years longer. As far as the amps are concerned, they seem to sound well enough on youtube (with its limitations) but I would like to hear a good recording of mildly distorted playing.
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