Hi folks
I want to build a pre-amp to accompany my DIY DAC and rather than starting from scratch, I figured I would buy a Cambridge Audio A1 integrated amp and modify it. These amps are dirt cheap and there are tons of them available.
Basically what I propose to do is to remove the power amp board (as marked in red below) and replace it with a new PSU and whatever pre-amp /buffer circuitry is necessary.
Once the power amp board is gone, there are only two bits of active circuitry left, which I have outlined in green below, I would probably remove the NE5532s, solder in 8 pin sockets and use different, more modern opamps, as well as upgrading the caps.
The great advantage of reworking one of these amps is not just cost, but the availability of the schematics and not having t worry about making an enclosure and buying and fitting the control pots, power switch etc - half the job is already done and there is a large volume of space to play with, you could fit a lot of PSU and other circuitry into the space available.
Just looking for opinions, thoughts, tips, guidance etc. maybe this is a dumb idea, or maybe there is a better approach?
I want to build a pre-amp to accompany my DIY DAC and rather than starting from scratch, I figured I would buy a Cambridge Audio A1 integrated amp and modify it. These amps are dirt cheap and there are tons of them available.
Basically what I propose to do is to remove the power amp board (as marked in red below) and replace it with a new PSU and whatever pre-amp /buffer circuitry is necessary.
Once the power amp board is gone, there are only two bits of active circuitry left, which I have outlined in green below, I would probably remove the NE5532s, solder in 8 pin sockets and use different, more modern opamps, as well as upgrading the caps.
The great advantage of reworking one of these amps is not just cost, but the availability of the schematics and not having t worry about making an enclosure and buying and fitting the control pots, power switch etc - half the job is already done and there is a large volume of space to play with, you could fit a lot of PSU and other circuitry into the space available.
Just looking for opinions, thoughts, tips, guidance etc. maybe this is a dumb idea, or maybe there is a better approach?
Attachments
This I have done in several ways some years ago. Most easy solution it is the creating of an interface between the output of exist preamp PCB and the input of the power amp.
Strongly better it is to remove all active preamp parts (except RIAA head amp) and using only the power amp IC TDA1514A as active part for driving a still to be retrofitted preamp female cinch output socket pair.
This means, only the input selector, attenuator for volume control and the power amp PCB's are in the signal path. Between power amp output and preamp sockets I would introduce each a 10 ohms resistor.
So you can use even long screening leads (cables) to your power amp without any disadvantages (often not possible with a typicall preamp without decrease the sound quality).
I recommend the reducing from the gain factor of 30 - go to
TDA1514A pdf, TDA1514A Szczegoły, TDA1514A dane, TDA1514A view ::: ALLDATASHEET :::
Strongly better it is to remove all active preamp parts (except RIAA head amp) and using only the power amp IC TDA1514A as active part for driving a still to be retrofitted preamp female cinch output socket pair.
This means, only the input selector, attenuator for volume control and the power amp PCB's are in the signal path. Between power amp output and preamp sockets I would introduce each a 10 ohms resistor.
So you can use even long screening leads (cables) to your power amp without any disadvantages (often not possible with a typicall preamp without decrease the sound quality).
I recommend the reducing from the gain factor of 30 - go to
TDA1514A pdf, TDA1514A Szczegoły, TDA1514A dane, TDA1514A view ::: ALLDATASHEET :::
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I'm going to be curt now
In short....why bother.
The preamp isn't anything special, and most folks do the opposite: remove the preamp and use them as power amps.
I own an A1 mk3 and annoyingly the preamp has gotten worse and worse. When I bought it several years ago there was slight DC output when he volume control is moved.
Volume goes up, cone moved out momentarily (1 sec), and vice versa.
I should probably fix this but for 30 GBP used, I'm probably end u just using as amp and removing the pre.
In short....why bother.
The preamp isn't anything special, and most folks do the opposite: remove the preamp and use them as power amps.
I own an A1 mk3 and annoyingly the preamp has gotten worse and worse. When I bought it several years ago there was slight DC output when he volume control is moved.
Volume goes up, cone moved out momentarily (1 sec), and vice versa.
I should probably fix this but for 30 GBP used, I'm probably end u just using as amp and removing the pre.
So removing the preamp parts is also a good idea, not surprising really, seeing as they are simple things we NE5532s.
I was thinking a Pass Labs B1 Buffer might make a good pre-amp circuit to replace the Cambridge one..
I'm running two Linn power amps bi-amping a pair of Linn Index speakers, in case that makes any difference.
I was thinking a Pass Labs B1 Buffer might make a good pre-amp circuit to replace the Cambridge one..
I'm running two Linn power amps bi-amping a pair of Linn Index speakers, in case that makes any difference.
Well I'm not going to slate the A1.
I've lived with one for many years now. I love many things about it. I've both passive crossover and active filters and may get another A1 as a power amp, so I have 4 channels TDA1514. I have 4 channels of LM4780 amps. I'm just missing the case and actual arrangement of it. And a comparison would be interesting.
It's great that as a DIY audio nut, when you open the A1 and find they made efforts to make the PCB silkscreen look the way it does. No-one does that. Who would see it? 🙄
But the front end is at least prone to ageing like anything I guess. And I suppose no all that high end. My biggest issue is the lack of tone defeat switch, so perhaps that could be a good upgrade. The amp itself sounds pleasantly warm, far less shrill than some HT receiver of the same era. Probably bested by a NAD of the same period (302?)
That's probably a case of the output end of things though. You may be hard pressed to improve upon the 5532 though. I have to admit I have thought of doing the same, and subbing in 4562 or something like that...but I haven't.
I've lived with one for many years now. I love many things about it. I've both passive crossover and active filters and may get another A1 as a power amp, so I have 4 channels TDA1514. I have 4 channels of LM4780 amps. I'm just missing the case and actual arrangement of it. And a comparison would be interesting.
It's great that as a DIY audio nut, when you open the A1 and find they made efforts to make the PCB silkscreen look the way it does. No-one does that. Who would see it? 🙄
But the front end is at least prone to ageing like anything I guess. And I suppose no all that high end. My biggest issue is the lack of tone defeat switch, so perhaps that could be a good upgrade. The amp itself sounds pleasantly warm, far less shrill than some HT receiver of the same era. Probably bested by a NAD of the same period (302?)
That's probably a case of the output end of things though. You may be hard pressed to improve upon the 5532 though. I have to admit I have thought of doing the same, and subbing in 4562 or something like that...but I haven't.
I had a NAD 310, nice amp, I think the circuitry was very similar to the 302. Sadly mine, like many 90's NADs, died. A friend had an A1, we both had Mission 731 speakers, I could never discern much difference, although the A1 might have been slightly more detailed and the NAD slightly more punch, not that I'm good at describing such things.
I'm very open to other suggestions for a cheap integrated to use as the basis for this project, I just picked the Cambridge Audios as they are so common and cheap and easy to modify with the separate pre and power boards and readily available schematics.
I'd love to use a vintage 70s amp as I like the silver aesthetics, but no idea what to look for, what can be modified etc.
I'm very open to other suggestions for a cheap integrated to use as the basis for this project, I just picked the Cambridge Audios as they are so common and cheap and easy to modify with the separate pre and power boards and readily available schematics.
I'd love to use a vintage 70s amp as I like the silver aesthetics, but no idea what to look for, what can be modified etc.
The problem with the A1 is - WHICH A1 is it.. there are about 3-4 different revisions of it, and the final one is dreadful.
After a bit of studying and reading, I've decided to go with a Pass Labs B1 Buffer.
I took the original schematic and redrew it so it's symmetrical an more like how I will lay it out on the matrix board:
I already have most of the parts, just need to but the transistors, some resistors and a pair of 10uF caps.
I'm using BHC Aerovox 2200uF, 40V ALT20A in place of the 15000uF electrolytics as they are really good caps and in the B1 manual, Nelson says anything from 1000uF, 25V and up is fine.
For the 1uF caps, I already have some Russian PIO ones, any reason not to use them?
10uF polyester film caps can be ridiculously expensive, what cheap alternatives would work? Russian PIO can be had in 10uF dirt cheap, any reason not to try those?
I took the original schematic and redrew it so it's symmetrical an more like how I will lay it out on the matrix board:
I already have most of the parts, just need to but the transistors, some resistors and a pair of 10uF caps.
I'm using BHC Aerovox 2200uF, 40V ALT20A in place of the 15000uF electrolytics as they are really good caps and in the B1 manual, Nelson says anything from 1000uF, 25V and up is fine.
For the 1uF caps, I already have some Russian PIO ones, any reason not to use them?
10uF polyester film caps can be ridiculously expensive, what cheap alternatives would work? Russian PIO can be had in 10uF dirt cheap, any reason not to try those?
Attachments
Looking at ebay, I found these four options for cheap 100uf caps that should be good quality, prices are for a pair inc postage:
Russian 10uf 160V Paper in Oil MBGO-2 - 4.57
Russian 10uf 35V K52-1 Tantalum-Silver-Palladium - 4.75
Arcotronics 10uF 400V C4G MKP+++ - 7.58ukp
WIMA 10uF 63V MKS4 - 8.44ukp
There are some dirt cheap polypropylene 'for audio' caps from China, but I'm dubious about their quality.
The Russians are obviously a lot cheaper, about the same price as the cheap Chinese dubious polypropylene ones, but if I have to spend a bit to get the right part for the job, I shall.
Russian 10uf 160V Paper in Oil MBGO-2 - 4.57
Russian 10uf 35V K52-1 Tantalum-Silver-Palladium - 4.75
Arcotronics 10uF 400V C4G MKP+++ - 7.58ukp
WIMA 10uF 63V MKS4 - 8.44ukp
There are some dirt cheap polypropylene 'for audio' caps from China, but I'm dubious about their quality.
The Russians are obviously a lot cheaper, about the same price as the cheap Chinese dubious polypropylene ones, but if I have to spend a bit to get the right part for the job, I shall.
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B1 has no gain if you are good with that. It also uses very hard to find and expensive 2SK170s. You might consider some very good Preamp designs that use more readily available parts like standard BJTs like BC550/560. It’s not that hard to make a very clean sounding one. Or if you must use JFETs, even a BF862 is still readily available even bough discontinued.
The zero gain is fine, or at least I think so, if no, the B1 buffer will come in handy in another project down the line and I'll build something else for my pre-amp.
I ordered these Toshiba 2SK170BLs from China, I hope they are authentic, they were certainly cheap:
5X 2SK170-BL K170BL 2SK170 3 Pins DIP Original TOSHIBA FET | eBay
I ordered these Toshiba 2SK170BLs from China, I hope they are authentic, they were certainly cheap:
5X 2SK170-BL K170BL 2SK170 3 Pins DIP Original TOSHIBA FET | eBay
I have ordered similar ones from China. They will work and can sound surprisingly good - but are not real if tested properly - as genuines are circa $60 for qnty 4. You can also use the LSK170 which is quite good too.
Hmm, I'll try em and see how they sound, if not great, I'll explore what NOS options are available. Thanks for the tip.
I went and ordered the Russian PIO 10uF caps, at least then I know they are authentic and good quality.
I started building the circuit up on a matrix board, the largest I had was 15x9cm and I thought it would all fit, but nope, it's not all gonna fit due to the size of the Russian caps, so I'll have to join on a second piece of board. Not the most elegant build but as long as it works.
I went and ordered the Russian PIO 10uF caps, at least then I know they are authentic and good quality.
I started building the circuit up on a matrix board, the largest I had was 15x9cm and I thought it would all fit, but nope, it's not all gonna fit due to the size of the Russian caps, so I'll have to join on a second piece of board. Not the most elegant build but as long as it works.
There's an interesting discussion about alternative transistors for the B1 here:
B1 JFET alternatives list
B1 JFET alternatives list
Does anyone know if the Fairchild BF256B would be suitable for the B1 buffer? The datasheet says the IDSS is 6 to 13 mA.
Here's the datasheet:
http://www.mouser.com/ds/2/149/bf256b-292516.pdf
I'm just not happy using Chinese fakes, makes me worry I'll go to all the bother of building the thing just to have the SQ undermined by nasty fake Chinese parts.
Here's the datasheet:
http://www.mouser.com/ds/2/149/bf256b-292516.pdf
I'm just not happy using Chinese fakes, makes me worry I'll go to all the bother of building the thing just to have the SQ undermined by nasty fake Chinese parts.
Input selector for input 1 vs 2. You can use BF862 and it will work quite well. BF862 is SOT23 package and 40 for $7 from eBay. Superb sounding JFET.
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