11-03-2017, 10:50 PM
It's interesting that those networks present such a low impedance to the cartridge to reduce the amplitude down to MM levels (nominally 5mV or thereabouts) while simultaneously undoing the following RIAA EQ by making a big filter with the resistance and self-capacitance of the cartridge. Simple, but elegant 
Regarding the GP42 circuit, I thought it was worth mentioning that the HF lift offered by R1/C1 is a technique I'm very familiar with in isolation. Going back to when I was a kid, using terrible DIY speakers in a tiny box room with awful acoustics, I learnt that I could alter the response of my amps to help the speakers to sound reasonable. I spent a lot of time looking at the various tone control circuits and distilling them to their most simple equivalent circuits, putting them between stages of amplification. The amps (I did many this way) still had conventional Baxandall-style tone controls, but the aim was to get it sounding OK with those at their centre positions. Basically, the filters amounted to a big midrange suck-circuit. I dread to think how these amps might sound on decent speakers today, but at the time, the system sounded great to me - I regard it as good (albeit naive) engineering. I would have been 12 or so. The arrival of my first CD player (when I was 16) caused me to re-evaluate all this, and concentrate on flat amplifiers and better crossovers in the 'speakers. However, the techniques learnt back then are still useful when trying to equalise a cheap drive unit - for an example, see here: http://www.markhennessy.co.uk/activespea...ronics.htm (and the next page in the article - http://www.markhennessy.co.uk/activespeaker/mods.htm - which include acoustic measurements of the drive unit). Of course, designers of transistor radios were no strangers to these techniques either.
A bit of a digression, but back to the GP42, there's just too many variables in my mind at the moment. When the treble is at "max" and the bass at "min", the amp is flat apart from the ~3dB treble lift that we should get from R1/C1. The tone controls are simple, non-Baxandall types, so the treble is cut-only, and the bass is lift-only. But we don't know how the loudspeaker performs, nor the cartridge. I might have a lead on this - it's looking likely that I'll be able to borrow a loudspeaker and take some measurements... Perhaps the whole machine. That should be interesting, and will help a conversation elsewhere, where some have stated that the GP42 "needs a tweeter". To save confusion, I'll perhaps leave all that for the other thread.
But back to the topic here, I wasn't sure how the cartridge would react into this load. It does seem to be a fairly high impedance load compared to many we've seen, so perhaps the loading effects would be minimal? Unfortunately, I'd need a test disc (and a GP42) to explore this further.
Earlier I mentioned the Bush SRP51. I had (and wrecked) one of these as a kid, and have since got another one from eBay. Lovely thing... The cartridge is a Sonotone 9TA HC.
This is a fairly neat little amplifier. It starts with a 1M volume pot, immediately followed by 100k. So ignoring whatever comes next, the input impedance varies from 1M to 91k.
What does come next is a CE preamp using a Darlington pair. Feedback is used for biasing, but with a pair of resistors (R1 and R3) between base and collector. The midpoint of these resistors supply the base via a 330k resistor, and is also bootstrapped via C2, further raising the input impedance.
It's hard to estimate the input impedance because it'll strongly depend on hfe, but let's imagine both transistors have a hfe of 200 each, so that's ~40,000 in combination. As collector current appears to be 1.5mA (0.7V/470), then re is 16.7 ohms, putting Zin of the transistors at ~790k. And RE (470R) is also magnified by hfe. So the overall circuit impedance - including the effect of bootstrapping - is hard to predict, but let's remember the 100k at the input (R18) and assume that the stuff following is so high that the R18 dominates. In short, input impedance is perhaps somewhere in the 90k region when the volume is at max. Perhaps turning the volume up provides some bass cut, which might well be useful...
Note R4/C4. These provide some HF lift by reducing the value of emitter resistance.
Also note R12/C9 in the NFB loop of the power amp. These provide bass boost.
Both these networks are there in part because the tone controls are loss networks. Overall, the levels of boost from these fixed networks are chosen to give the tone controls sensible range, and to compensate for the response of both the cartridge and the loudspeaker. But it's interesting to note that the output of the power amp is picked up by R15/17 and fed to the "Tape" socket. This means the recording will be affected by the settings of the tone controls, and will have any EQ needed to make the speaker sound OK. When playing back the recordings, the sound will travel through the amplifier for a second time, so will be double-equalised!
So anyway, that's an example of a reasonably high (for a transistor circuit) input impedance.
Another example is the Hacker GP45, which uses the ACOS GP104. That one works into a 100k volume pot followed by a 2-transistor preamp that looks to have a "medium" input impedance; rather less than the Bush. Perhaps "high-10s" of k rather than 100s...
What's interesting about this one is the lack of EQ. With the Baxandall tone controls in their centre position, the amp is ruler-flat across the audio band. The test specification involves verifying this, then going on to verify the response with a test disc, which shows the the cartridge does produce a pretty flat response working into the ~100k load. My example has been up in the loft for some 5+ years, and needs a lot of work, so I can't say how it sounds with the tone controls in their centre position, but I wouldn't be surprised if they need to be turned up a tad. There's +/-12dB of range from them, so plenty compared to the simpler networks in the GP42/SRP51.
In the course of this, I stumbled across this very interesting and informative thread: https://www.vintage-radio.net/forum/show...hp?t=60957 - some familiar faces there
As I said earlier, I abandoned ceramic cartridges when I was 12, so this has been very interesting for me. Especially the diversity of approaches now compiled in this thread. Something of an understatement I know, but the fact you can get such a good frequency response so easily is very fortunate for the equipment manufacturers who don't need to mess about with RIAA networks! Hard to imagine how the classic 2-valve player would have been feasible if it wasn't for the magic of ceramic
Cheers,
Mark
Regarding the GP42 circuit, I thought it was worth mentioning that the HF lift offered by R1/C1 is a technique I'm very familiar with in isolation. Going back to when I was a kid, using terrible DIY speakers in a tiny box room with awful acoustics, I learnt that I could alter the response of my amps to help the speakers to sound reasonable. I spent a lot of time looking at the various tone control circuits and distilling them to their most simple equivalent circuits, putting them between stages of amplification. The amps (I did many this way) still had conventional Baxandall-style tone controls, but the aim was to get it sounding OK with those at their centre positions. Basically, the filters amounted to a big midrange suck-circuit. I dread to think how these amps might sound on decent speakers today, but at the time, the system sounded great to me - I regard it as good (albeit naive) engineering. I would have been 12 or so. The arrival of my first CD player (when I was 16) caused me to re-evaluate all this, and concentrate on flat amplifiers and better crossovers in the 'speakers. However, the techniques learnt back then are still useful when trying to equalise a cheap drive unit - for an example, see here: http://www.markhennessy.co.uk/activespea...ronics.htm (and the next page in the article - http://www.markhennessy.co.uk/activespeaker/mods.htm - which include acoustic measurements of the drive unit). Of course, designers of transistor radios were no strangers to these techniques either.
A bit of a digression, but back to the GP42, there's just too many variables in my mind at the moment. When the treble is at "max" and the bass at "min", the amp is flat apart from the ~3dB treble lift that we should get from R1/C1. The tone controls are simple, non-Baxandall types, so the treble is cut-only, and the bass is lift-only. But we don't know how the loudspeaker performs, nor the cartridge. I might have a lead on this - it's looking likely that I'll be able to borrow a loudspeaker and take some measurements... Perhaps the whole machine. That should be interesting, and will help a conversation elsewhere, where some have stated that the GP42 "needs a tweeter". To save confusion, I'll perhaps leave all that for the other thread.
But back to the topic here, I wasn't sure how the cartridge would react into this load. It does seem to be a fairly high impedance load compared to many we've seen, so perhaps the loading effects would be minimal? Unfortunately, I'd need a test disc (and a GP42) to explore this further.
Earlier I mentioned the Bush SRP51. I had (and wrecked) one of these as a kid, and have since got another one from eBay. Lovely thing... The cartridge is a Sonotone 9TA HC.
This is a fairly neat little amplifier. It starts with a 1M volume pot, immediately followed by 100k. So ignoring whatever comes next, the input impedance varies from 1M to 91k.
What does come next is a CE preamp using a Darlington pair. Feedback is used for biasing, but with a pair of resistors (R1 and R3) between base and collector. The midpoint of these resistors supply the base via a 330k resistor, and is also bootstrapped via C2, further raising the input impedance.
It's hard to estimate the input impedance because it'll strongly depend on hfe, but let's imagine both transistors have a hfe of 200 each, so that's ~40,000 in combination. As collector current appears to be 1.5mA (0.7V/470), then re is 16.7 ohms, putting Zin of the transistors at ~790k. And RE (470R) is also magnified by hfe. So the overall circuit impedance - including the effect of bootstrapping - is hard to predict, but let's remember the 100k at the input (R18) and assume that the stuff following is so high that the R18 dominates. In short, input impedance is perhaps somewhere in the 90k region when the volume is at max. Perhaps turning the volume up provides some bass cut, which might well be useful...
Note R4/C4. These provide some HF lift by reducing the value of emitter resistance.
Also note R12/C9 in the NFB loop of the power amp. These provide bass boost.
Both these networks are there in part because the tone controls are loss networks. Overall, the levels of boost from these fixed networks are chosen to give the tone controls sensible range, and to compensate for the response of both the cartridge and the loudspeaker. But it's interesting to note that the output of the power amp is picked up by R15/17 and fed to the "Tape" socket. This means the recording will be affected by the settings of the tone controls, and will have any EQ needed to make the speaker sound OK. When playing back the recordings, the sound will travel through the amplifier for a second time, so will be double-equalised!
So anyway, that's an example of a reasonably high (for a transistor circuit) input impedance.
Another example is the Hacker GP45, which uses the ACOS GP104. That one works into a 100k volume pot followed by a 2-transistor preamp that looks to have a "medium" input impedance; rather less than the Bush. Perhaps "high-10s" of k rather than 100s...
What's interesting about this one is the lack of EQ. With the Baxandall tone controls in their centre position, the amp is ruler-flat across the audio band. The test specification involves verifying this, then going on to verify the response with a test disc, which shows the the cartridge does produce a pretty flat response working into the ~100k load. My example has been up in the loft for some 5+ years, and needs a lot of work, so I can't say how it sounds with the tone controls in their centre position, but I wouldn't be surprised if they need to be turned up a tad. There's +/-12dB of range from them, so plenty compared to the simpler networks in the GP42/SRP51.
In the course of this, I stumbled across this very interesting and informative thread: https://www.vintage-radio.net/forum/show...hp?t=60957 - some familiar faces there
As I said earlier, I abandoned ceramic cartridges when I was 12, so this has been very interesting for me. Especially the diversity of approaches now compiled in this thread. Something of an understatement I know, but the fact you can get such a good frequency response so easily is very fortunate for the equipment manufacturers who don't need to mess about with RIAA networks! Hard to imagine how the classic 2-valve player would have been feasible if it wasn't for the magic of ceramic
Cheers,
Mark
