Hi ! the idea is to use the design for a line stage ... so gain voltage of 3-4 times max and load 10K and up ...
As i am focusing only the output stage of the ZD22 i have to reduce the value of the feedback resistor to get the gain of 3-4 (I am cutting out the input amp stage and phono stage ... i do not listen to LPs anymore ... only their digital copies at max)
A friend of mine has already done that with no issue One thing he told me is that changing the feedback resistor for an Holco opened up the sound a lot. Even the small details count.
My parts selection will be limited by the bjt models available because i really want to simulate first the circuit, given that is very simple it could be a nice learning step. And see the impact of gain, voltage supply, different bjts, and so on ... Once i get very good distortion and noise graph i will start building a prototype ... that is the plan
Thanks a lot for the very valuable advice. Changing topology is absolutely out of question. I am sure this already good design can provide very very good sound when optimized. So better spend time optimizing a very simply topology than going the complexity route. The scope is to get an excellent gain stage usable as line amp/buffer with gain with just two bjts.
Very interesting and i am sure that your project is much more refined. However as i said i have an open issue with this circuit. I have read so many people praising the sound of the original that can be even not optimized for just line preamp duties. I have to find a source for bjt models and start simulating. I will ask about that Thanks a lot again.
Last edited:
Here's a little sim for a Gv = 4 circuit. You need Bob Cordell's models for the BC550, the BC556 model ships with LTspice I think. It's also right at the BC550's Vce limit with a 55 V supply - the output side is less critical.
It appears that the output side will benefit greatly from transistors with high Early voltage Va and low Cob. The likes of 2N5401, 2SA1514K or MPS8599 should work well. Note: Beware of transistor models with "dummy" VAF, such as exactly 100 V. Also note that varying DC bias you may find a "sweet spot" in distortion that may be largely cancellation of 2nd harmonic and not actually as significant in real life.
You can take some voltage off of the input transistor by adding two extra components, but beware that your optimum emitter resistor value is likely to shift. That will also depend on the frequency you're looking at, as towards 20 kHz you increasingly run into slew rate issues and slew rate is also influenced by this resistor.
It appears that the output side will benefit greatly from transistors with high Early voltage Va and low Cob. The likes of 2N5401, 2SA1514K or MPS8599 should work well. Note: Beware of transistor models with "dummy" VAF, such as exactly 100 V. Also note that varying DC bias you may find a "sweet spot" in distortion that may be largely cancellation of 2nd harmonic and not actually as significant in real life.
You can take some voltage off of the input transistor by adding two extra components, but beware that your optimum emitter resistor value is likely to shift. That will also depend on the frequency you're looking at, as towards 20 kHz you increasingly run into slew rate issues and slew rate is also influenced by this resistor.
I don't understand. The original topology uses seven BJTs. If you're only going to use two, there isn't anything left of the original topology except a CFP with gain.
Hi ! thank you very much indeed for your precious support.
My main object of interest is the output stage only.
As i said before i am very uneducated in electronics ... but i would like really to select the best parts (i.e. active and passive) for the job.
My point is:
1) i really do not know how to do it
2) i am afraid that specific models for those best parts will not be available ?
I am still stuck on point 1.
i wonder if a simulation can reveal all this ... if not it will be a big problem.
Again the circuit employs only one NPN and one PNP.
When i see this i feel lost ... the choice is almost endless ...
Transistors | Products | Toshiba Electronic Devices & Storage Corporation | Americas – United States
maybe these could work ?
Transistors | Products | Toshiba Electronic Devices & Storage Corporation | Americas – United States
for the PNP part that i remember gets hotter i would prefer a part easy to heatsink properly. And more powerful ... like this one
HTTP 301 This page has been moved
I really believe in the CFP topology because it was very popular in the 70s in high end preamp like also a Kenwood 700C for the line buffer block
With the best parts now available, with the right simulation i think a new circuit could outperform even the original 😱
But between to say and to actually do there is a sea ... we use to say here
i am not that good at sailing ... 😱
Thanks a lot again
Hi ! yes indeed ... that is just what is needed for a line preamp. 😱
If there is one thing that i do not need at all are tone controls ... and i understand the tone controls are the reason for this complicated schematic ?
by the way .... it seems that CFP is not that popular these days 🙄
Well there are always better topologies, even within the CFP world, for example a CFP with CCS, but I'm struggling to see what all this really has to do with the Radford topology, which is indeed 2/3 about driving a tone control stage.
Hi ! thanks very interesting. May i ask in which sense they are better ? lower in distortion maybe ?
yes ... i am just interested in the othe 1/3 that drives the power amp stage. That is my object of interest. A very simple but good sounding line stage.
I have seen line stage with tenths of active parts and even more passive parts.
Is all that stuff really needed ? for what ? is it not a case of overengineering ?
why not optimizing simple topologies to their limits instead of take the complexity route ? a KISS principle application ...
I am just pruning out what is unnecessary for me ... like tone controls.
Last edited:
Hallo folks,
This thread is already quite old, but perhaps I may share my recent experiences. Just for fun, I have made a clone of the Radford ZD22 preamp. My clone contains three PCBs:
1. Exact copy of the "Disc Amplfier Module", using 2N3964, BC107B, BC109C, 2N2219A and 2N2905A (but without tape buffer, since I did not need this).
2. Exact copy of the "Power Supply Module ZD22", but using other transistors (H945, SF129D, BC108, 2SC4793, MPSA06, MPSA92 for TS43...48).
3. Own version of the "Line Amplifier Module", including only the final two transistor stage (tone control stage and line input amplifier omitted). As transistors I used 2SC1775 and SF118.
The contents of my junkbox dictated the use of some rather obscure transistors from the former German Democratic Republic (SF129D and SF118).
AC power is supplied by an external transformer (which was salvaged from a defunct Marantz CD80 CD player...).
Initial listening tests showed that the ZD22 clone sounds subjectively excellent. I am quite happy with it.
If anyone is interested I can share some pictures and also printed-circuit board layouts in Sprint Layout (*.lay6) format.
Best regards,
Aren van Waarde
Groningen Area, The Netherlands
This thread is already quite old, but perhaps I may share my recent experiences. Just for fun, I have made a clone of the Radford ZD22 preamp. My clone contains three PCBs:
1. Exact copy of the "Disc Amplfier Module", using 2N3964, BC107B, BC109C, 2N2219A and 2N2905A (but without tape buffer, since I did not need this).
2. Exact copy of the "Power Supply Module ZD22", but using other transistors (H945, SF129D, BC108, 2SC4793, MPSA06, MPSA92 for TS43...48).
3. Own version of the "Line Amplifier Module", including only the final two transistor stage (tone control stage and line input amplifier omitted). As transistors I used 2SC1775 and SF118.
The contents of my junkbox dictated the use of some rather obscure transistors from the former German Democratic Republic (SF129D and SF118).
AC power is supplied by an external transformer (which was salvaged from a defunct Marantz CD80 CD player...).
Initial listening tests showed that the ZD22 clone sounds subjectively excellent. I am quite happy with it.
If anyone is interested I can share some pictures and also printed-circuit board layouts in Sprint Layout (*.lay6) format.
Best regards,
Aren van Waarde
Groningen Area, The Netherlands
Just to add something to Jens Langvad Low Dist. Oscillator LDO3 story:
Hi ! thank you very much for your very interesting post
Did you performed any THD+noise measurement on your creation ?
If i remember well the Line Amplifier Stage should have a V gain of about 6 times a little too much for a line stage
I think i run some sims and when wired as just a buffer without gain it showed slightly better performance in terms of THD ?
i have to look for the files
Thanks a lot again
Hi Ginetto,
No, I did not perform any THD+noise measurements. I examined all gain stages with a sine/squarewave generator and an oscilloscope. All scope tests proved OK. And I performed initial listening tests, using the following associated equipment:
Turntable: Sony PS-X3 (heavy direct-drive) with Shure M91-MG-D cartridge (rather cheap, came from a Dual turntable that was owned by my uncle)
Power amplifier: Van Medevoort PA222 (100W/channel, high quality)
Loudspeakers: L'Audiophile La Petite (homemade)
The preamp is very quiet and does not produce any audible hum and noise when it receives no cartridge signal.
The original Radford folder claimed a signal-to-noise ratio of -70dBv at rated sensitivity (2 mV MD input), -78dBv at 5mV in.
This may be true, even with my ear close to the speakers the preamp is very quiet.
Your memory is correct, the line amplifier stage has a voltage gain of 6.7 x.
Since I intend to use the Radford for listening to LP records, I need some voltage gain in the line stage as the RIAA preamp stage has only 50 x gain (at 1 kHz).
Even with the simple M91 cartridge, the Radford makes wonderful music.
Best regards,
Aren
No, I did not perform any THD+noise measurements. I examined all gain stages with a sine/squarewave generator and an oscilloscope. All scope tests proved OK. And I performed initial listening tests, using the following associated equipment:
Turntable: Sony PS-X3 (heavy direct-drive) with Shure M91-MG-D cartridge (rather cheap, came from a Dual turntable that was owned by my uncle)
Power amplifier: Van Medevoort PA222 (100W/channel, high quality)
Loudspeakers: L'Audiophile La Petite (homemade)
The preamp is very quiet and does not produce any audible hum and noise when it receives no cartridge signal.
The original Radford folder claimed a signal-to-noise ratio of -70dBv at rated sensitivity (2 mV MD input), -78dBv at 5mV in.
This may be true, even with my ear close to the speakers the preamp is very quiet.
Your memory is correct, the line amplifier stage has a voltage gain of 6.7 x.
Since I intend to use the Radford for listening to LP records, I need some voltage gain in the line stage as the RIAA preamp stage has only 50 x gain (at 1 kHz).
Even with the simple M91 cartridge, the Radford makes wonderful music.
Best regards,
Aren
Hi Arem thank you very much for your kind and interesting reply
i listen to digital sources only
A friend of mine made a small prototype on a perforated board of just the last line output stage after the main volume control We liked the sound a lot
Then he switched to tubes and stopped the project
The Sziklai pair can be found in preamps of the 70s like this one said to sound very decent as well
https://www.hifiengine.com/manual_library/onkyo/p-303.shtml
Imhe the volume control quality is important I love stepped attenuators but the good ones can be very expensive
Congratulations again 🙂👍
i listen to digital sources only
A friend of mine made a small prototype on a perforated board of just the last line output stage after the main volume control We liked the sound a lot
Then he switched to tubes and stopped the project
The Sziklai pair can be found in preamps of the 70s like this one said to sound very decent as well
https://www.hifiengine.com/manual_library/onkyo/p-303.shtml
Imhe the volume control quality is important I love stepped attenuators but the good ones can be very expensive
Congratulations again 🙂👍
Final verdict on my "simplified Radford ZD22" clone (after listening to more than 100 LP records, turntable is now a Pioneer PLX-1000 with a vintage Shure M91-MG-D):
1. Imaging is very stable and precise, soundstage is wide but not very deep (this may also be related to the properties of the moving magnet cartridge).
2. Dynamics and transient response are VERY good, even on heavily modulated LP tracks there is no audible overload. Many records that I did not appreciate much in the past now sound quite "alive".
3. One can listen to several records in series, the audio chain doesn't cause any listener fatigue. Thus, I love this ancient pre!
I can strongly recommend every analog freak to build a copy of this preamp. Here is a look at the innards of my own clone (RIAA section left, line stage center, PSU right, transformer external):
1. Imaging is very stable and precise, soundstage is wide but not very deep (this may also be related to the properties of the moving magnet cartridge).
2. Dynamics and transient response are VERY good, even on heavily modulated LP tracks there is no audible overload. Many records that I did not appreciate much in the past now sound quite "alive".
3. One can listen to several records in series, the audio chain doesn't cause any listener fatigue. Thus, I love this ancient pre!
I can strongly recommend every analog freak to build a copy of this preamp. Here is a look at the innards of my own clone (RIAA section left, line stage center, PSU right, transformer external):
Hi AS audio,
Actually I didn't listen to LP records for many years, thus I haven't made any direct comparison between various preamps. In the past or remote past, I have used various turntables (Thorens, Rotel, Sony, Pioneer) and various cartridges (both MM and MC) with homemade electronics. I remember that I used: Elektor Prelude (1983), A&T P832 (1983), and various ETI preamps from the 1980s (Elektor, A&T and ETI were DIY audio magazines published in the Netherlands). Since I was not a very experienced electronics constructor in the 1980s and early 1990s, results of my copies of these designs may have been suboptimal. One discrete op amp-based pre from ETI showed unpredictable stability problems which could not be cured, even after the implementation of a few modifications that were proposed by the journal. Undesired HF oscillations in RIAA and/or line-level preamps may be more common than most people think, and may result in poor sound and imaging instability. An MD preamp with pencil tubes and SRPP gain stages that I designed myself was probably also not unconditionally stable. With MC cartridges, my preamps had rather poor signal-to-noise (besides some hiss they showed often also audible hum). Audible overload also occurred in some cases. But the Jens Langvad design does everything well! No "audiophilia nervosa" on my part in this case...
Regarding the "East German caps" (e.g. the big yellow cylinders in the line stage): they are MKC and MKT types. After the fall of the Berlin Wall (thus, in the early 1990s), huge amounts of electronic parts from the former GDR were dumped on the West German and Dutch markets, and were sold for ridiculously low prices. An electronics store in my home town (Groningen) used to sell such parts by weight (1 kilogram of mixed parts for 5 guilders). Conrad Electronics sold them too. Most home constructors didn't want to use these parts since they were bulky and in their eyes suspect. I bought several kilograms at the local store (until they were all gone) and stored them in my attic. Some firms that sell ex-military and ex-navy equipment may still have a small stock of ex-GDR parts.
Hope this satisfies your curiosity!
Actually I didn't listen to LP records for many years, thus I haven't made any direct comparison between various preamps. In the past or remote past, I have used various turntables (Thorens, Rotel, Sony, Pioneer) and various cartridges (both MM and MC) with homemade electronics. I remember that I used: Elektor Prelude (1983), A&T P832 (1983), and various ETI preamps from the 1980s (Elektor, A&T and ETI were DIY audio magazines published in the Netherlands). Since I was not a very experienced electronics constructor in the 1980s and early 1990s, results of my copies of these designs may have been suboptimal. One discrete op amp-based pre from ETI showed unpredictable stability problems which could not be cured, even after the implementation of a few modifications that were proposed by the journal. Undesired HF oscillations in RIAA and/or line-level preamps may be more common than most people think, and may result in poor sound and imaging instability. An MD preamp with pencil tubes and SRPP gain stages that I designed myself was probably also not unconditionally stable. With MC cartridges, my preamps had rather poor signal-to-noise (besides some hiss they showed often also audible hum). Audible overload also occurred in some cases. But the Jens Langvad design does everything well! No "audiophilia nervosa" on my part in this case...
Regarding the "East German caps" (e.g. the big yellow cylinders in the line stage): they are MKC and MKT types. After the fall of the Berlin Wall (thus, in the early 1990s), huge amounts of electronic parts from the former GDR were dumped on the West German and Dutch markets, and were sold for ridiculously low prices. An electronics store in my home town (Groningen) used to sell such parts by weight (1 kilogram of mixed parts for 5 guilders). Conrad Electronics sold them too. Most home constructors didn't want to use these parts since they were bulky and in their eyes suspect. I bought several kilograms at the local store (until they were all gone) and stored them in my attic. Some firms that sell ex-military and ex-navy equipment may still have a small stock of ex-GDR parts.
Hope this satisfies your curiosity!
The trouble is there are too many things to be optimized than a simple topology can handle - it can never perform as well as a complex circuit where each thing is addressed. Individual active components are non-ideal by a large margin in practical situations. Compare to optics where a precision wide-aperture lens requires lots of elements to reduce the 7 optical distortions to required limits - no matter how much you optimize a single-element lens its woeful by comparison, there are not enough degrees of freedom.
Simple is very seldom the best approach for analog precision. For instance a cascoded degenerated common-emitter circuit driving a constant current source load can be much more linear than any of the transistors that it is built from. Or adding a buffer between circuit segments can improve impedance issues for both of them. Or adding an EB diode to protect an input transistor from damaging reverse-bias from switching transients... More complex, better performance, more robust. Up to a point of course...
In electronic devices the individual device behaviours have considerable spread, and drift over time and with temperature, making precise cancellation-like optimization unworkable, and feedback is usually required for performance (whether local or global or both), which means stability has to be addressed, and then you have to ensure latch-up-free startup, sensible clipping recovery, etc etc, which may all benefit from extra components...
The only other approach to analog performance is make the signal smaller (relatively), by using higher voltage and higher power components, but that tends to cost more than the "complex" design, and create other undesirable issues. Compare vehicle suspension - you can just make the wheels 10 times larger to get a smoother ride, or you can do the "complex" design work with normal wheel sizes using springs, shocks, hydro-dynamics, subframes etc.
Until someone invents a really linear discrete active device we can live with opamps as our nicely performing packaged complexity...
This thread is about ZD22 preamp and Radford (transistor gear) in general.
I appreciate your philosphic remarks but would prefer to discuss the circuit of
this product as is and hint at weak points or ways of improving it. It surely is
no longer state of the art some 50 years later, but many people want to learn
from old masters of the past. Opamps are boring.
So, as a question to aren, which schematic did you use ? The thread deserves
a direct link or a picture.
I appreciate your philosphic remarks but would prefer to discuss the circuit of
this product as is and hint at weak points or ways of improving it. It surely is
no longer state of the art some 50 years later, but many people want to learn
from old masters of the past. Opamps are boring.
So, as a question to aren, which schematic did you use ? The thread deserves
a direct link or a picture.
Dear "AS Audio", the original service manual for the Radford HD250-ZD22 can be downloaded from several websites. In a follow-up message, I can post links.
To this message I attach schematics of the RIAA preamp stage (gain 50), line stage (gain 6.7) and PSU (output 70V) that I have built. In some cases, I used original transistors, in other cases replacements. All modifications are described in the posted schematics.
To this message I attach schematics of the RIAA preamp stage (gain 50), line stage (gain 6.7) and PSU (output 70V) that I have built. In some cases, I used original transistors, in other cases replacements. All modifications are described in the posted schematics.
Here are some links. The original HD250-ZD22 service manual can be downloaded from: audiocircuit.dk, elektrotanya.com and radiomuseum.org (to mention just a few).
In my previous message, I forgot to say that in the line stage, I replaced the original 6.8 µF electrolytics by foil capacitors. For C5, I used 4.7 µF MKC and C3 was 10 µF MKT (C5 and C3 are the yellow cylinders that are visible in my posted picture of the innards of the preamp).
In my previous message, I forgot to say that in the line stage, I replaced the original 6.8 µF electrolytics by foil capacitors. For C5, I used 4.7 µF MKC and C3 was 10 µF MKT (C5 and C3 are the yellow cylinders that are visible in my posted picture of the innards of the preamp).