and now for something completely different part 3

[kevin gilmore]

There was a cement power resistor tit perfectly, I am not in a position to get the part number now.

you need the balanced front end board which stacks on top of the amp board if you want to do balanced.

the +/- reg inputs come from GRLV supplies set for the appropriate voltage (which I don't remember)

I don't remember what the pot does, probably output current adjust

single ended its 25 watts pure class A into 8 ohms. Balanced 100 watts pure class A into 8 ohms.

you can raise the power supply voltages and lower the idle current and get 400 watts into my 8 ohms 50 watts of which are class A

Obviously you need the appropriate mound of aluminum to get rid of the heat. Or water cooling.

[jose]

Trimpot is for output current adjust. I used +/- 23Vdc but you can use until +/- 24Vdc.

If you are thinking of using just one trafo for the four channels you will need a huge trafo. For a balanced configuration I preferred to use two PSU's and two GRLV's

Edit: I'm sorry, I was confused. I still have trouble distinguishing the names of these two amplifiers

 

Edited June 19, 2018 by jose

[lkong]

16 hours ago, kevin gilmore said:

There was a cement power resistor tit perfectly, I am not in a position to get the part number now.

 you need the balanced front end board which stacks on top of the amp board if you want to do balanced.

the +/- reg inputs come from GRLV supplies set for the appropriate voltage (which I don't remember)

I don't remember what the pot does, probably output current adjust

single ended its 25 watts pure class A into 8 ohms. Balanced 100 watts pure class A into 8 ohms.

you can raise the power supply voltages and lower the idle current and get 400 watts into my 8 ohms 50 watts of which are class A

Obviously you need the appropriate mound of aluminum to get rid of the heat. Or water cooling.

I think congo5 used xicon, which you will have to bend the legs. That's fine for me.

1 hour ago, jose said:

Trimpot is for output current adjust. I used +/- 23Vdc but you can use until +/- 24Vdc.

 If you are thinking of using just one trafo for the four channels you will need a huge trafo. For a balanced configuration I preferred to use two PSU's and two GRLV's

  

  

I believe the Uberamp drive board runs on +/- 40Vdc. It can certainly run on lower rails.

It will be a two chassis solution in my case.

I was inspired by Benchmark AHB2. Is it a good idea to use SMPS for the unreg power supply? Like this one: http://connexelectronic.com/product/smps800re/

+/- 50 for unreg, and GRLV drops it to +/- 40 for reg.

 

Edited June 18, 2018 by lkong

[kevin gilmore]

I guess you can use the amps supply at +/-40 and then you don't populate the reg transistors.

But I would go with +/-55v and then use the reg transistors and the GRLV to make the 40v references

you might get away with +/-50v because the voltage drop of the GRLV should be minimal 

that way it's going to be absolutely noise free.

congo5 did have some trouble with the power on, the switch on for the transformers he is using drew something like 25 amps

If that smps supply is designed 

 

[congo5]

There are resistors that fit better but I saved a lot of money by getting 100 Xicon's

Yes its a very tight fit

 

That SMPS looks interesting to me and its current limited to 1050W, would add the fan for sure.

I would like to try it, with two 50v feeding the unreg and with the GRLV set to 40v or slightly lower if using balanced as I don't really need 640 Watts RMS class AB

 

been wanting to take mine apart and make it into mono blocks that could be used as stereo or balanced and so it wouldn't weigh 80LBS

also need to double the size of heatsinks or add water cooling

 

And Yes I had trouble on startup with 1000va of transformer feeding close to 200,000uf of caps

the current inrush blew  12 35A diode bridges and the resulting short to ground blew up many of my soft start components

[lkong]

1 hour ago, congo5 said:

There are resistors that fit better but I saved a lot of money by getting 100 Xicon's 

Yes its a very tight fit

  

That SMPS looks interesting to me and its current limited to 1050W, would add the fan for sure.

I would like to try it, with two 50v feeding the unreg and with the GRLV set to 40v or slightly lower if using balanced as I don't really need 640 Watts RMS class AB

  

been wanting to take mine apart and make it into mono blocks that could be used as stereo or balanced and so it wouldn't weigh 80LBS

also need to double the size of heatsinks or add water cooling

 

And Yes I had trouble on startup with 1000va of transformer feeding close to 200,000uf of caps

the current inrush blew  12 35A diode bridges and the resulting short to ground blew up many of my soft start components

25amp draw that's for 4 boards or 2 boards?

If it's 25amp for 2 boards, the 800W PS should do 2 boards nicely. If not there's always 2kW ones.

Edited June 19, 2018 by lkong

[congo5]

I don't know about the 25A draw

not sure / don't remember

inrush was above that judging by damage at times

should have measured it but didn't

the average magnetic 15A circuit breaker might take 80A to trip instantly

I sometimes see startup amps on a 3ton A/C go over 110A on 25A fuses and 30A breakers

 

was using 4 boards

it was two years ago

think 5A running on 120v circuit or about 600W at idle from the wall   

and that's with a low bias of 1A per board 1/4A per pair, 150mv each 0.47R emitter resistor

 

Yes that 90%  efficienct 800W P/S looks right to me for two boards.

It depends on your speakers, 8ohms? 4ohoms?

how much power they and you require.

How much Bias / class A power you plan on

if balanced each board only sees half the load so an 8R becomes 4R when calculating Class A or power supply needs

 

 

[kevin gilmore]

The 25 amps was ac line input current rush

[congo5]

Yea OK thanks, enough amps to kill the cheep bridges I was trying to use

I forget things that don't come up, but hey I'm OLD

Edited June 19, 2018 by congo5

[lkong]

1 hour ago, congo5 said:

I don't know about the 25A draw

not sure / don't remember

inrush was above that judging by damage at times

should have measured it but didn't

the average magnetic 15A circuit breaker might take 80A to trip instantly

I sometimes see startup amps on a 3ton A/C go over 110A on 25A fuses and 30A breakers

 

was using 4 boards

it was two years ago

think 5A running on 120v circuit or about 600W at idle from the wall   

and that's with a low bias of 1A per board 1/4A per pair, 150mv each 0.47R emitter resistor

 

Yes that 90%  efficienct 800W P/S looks right to me for two boards.

It depends on your speakers, 8ohms? 4ohoms?

how much power they and you require.

How much Bias / class A power you plan on

if balanced each board only sees half the load so an 8R becomes 4R when calculating Class A or power supply needs

 

 

Planning to use the amp to drive my Quad ESL 63. Somewhere there might be a pair of Magnepan 3.7 with my name on it, but that's down the road.

[kevin gilmore]

In balanced mode the output current capability can blow the protection circuit of the esl63 if you attempt to play it to loud.

other than that, it should sound wonderful.

[BubbaMc]

Hi,

Could someone please direct me to the documentation for this amp? (schematic/bom etc)

Thanks!

[Pars]

Documentation? heh

Which amp? This is a confusing thread because there are at least 3 amps discussed here, maybe more (Kevin or others, correct me if I am wrong):

1. CFA2 (CFP) - current feedback amp. Each board does 1 channel SE, so 2 for stereo SE, 4 for stereo balanced. Kevin later did a mod to this adding current mirrors feeding the servo. People who had built the original noticed that the servo was audible as to quality of opamp. The schematic in Kevin's archive is the older version.
2. CFA3 - current feedback amp for balanced. Not sure if anyone ran boards for this one. Basically a pair of CFA2 amps and ubal-bal board on one pcb. Schematic does not show the current mirror mod as in 1 above
3. uberamp - seems to be a power amp; I've never quite figured this one out

Here are the schematics for the CFA2 (current mirror version), and the CFA3. I also attached my BOM for a CFA2. Amp quantity shown is for stereo SE. If going balanced, double it. It should reflect the later current mirror version. You'll also need some GRLVs or other power supplies, etc.

cfa2cmirror.pdf

cfa3production.PDF

CFP2_BOM_update2018march.xlsx

EDIT: I also ran across another schematic on Kevin's site which I hadn't seen before: currentsourceamp2.PDF. This seems very similar to the cfa2cmirror schematic, but is described as a transconductance amp.

Edited July 4, 2018 by Pars

[jose]

add an electrostat CFA

[BubbaMc]

On 7/4/2018 at 10:22 PM, Pars said:

Documentation? heh

Which amp? This is a confusing thread because there are at least 3 amps discussed here, maybe more (Kevin or others, correct me if I am wrong):

1. CFA2 (CFP) - current feedback amp. Each board does 1 channel SE, so 2 for stereo SE, 4 for stereo balanced. Kevin later did a mod to this adding current mirrors feeding the servo. People who had built the original noticed that the servo was audible as to quality of opamp. The schematic in Kevin's archive is the older version.
2. CFA3 - current feedback amp for balanced. Not sure if anyone ran boards for this one. Basically a pair of CFA2 amps and ubal-bal board on one pcb. Schematic does not show the current mirror mod as in 1 above
3. uberamp - seems to be a power amp; I've never quite figured this one out

Here are the schematics for the CFA2 (current mirror version), and the CFA3. I also attached my BOM for a CFA2. Amp quantity shown is for stereo SE. If going balanced, double it. It should reflect the later current mirror version. You'll also need some GRLVs or other power supplies, etc.

cfa2cmirror.pdf

cfa3production.PDF

CFP2_BOM_update2018march.xlsx

EDIT: I also ran across another schematic on Kevin's site which I hadn't seen before: currentsourceamp2.PDF. This seems very similar to the cfa2cmirror schematic, but is described as a transconductance amp.

Thanks very much for this. 

Is Kevin's site still active? If so could you post the link? All the old links are not working for me. Thanks again.

[lkong]

13 hours ago, BubbaMc said:

Thanks very much for this. 

Is Kevin's site still active? If so could you post the link? All the old links are not working for me. Thanks again.

All in those two google drives:

https://drive.google.com/drive/folders/0B7egryukiT7_TFlEQlBRejdVdDQ

https://drive.google.com/drive/folders/0B_iJFfZStuVhSE5nOHBVdTByR1k

[ti5002000]

Can i use a 500 mw CMF55 in the 1 ohm position,if i only 4 pin xlr in the output?

[Ariamella]

On 7/4/2018 at 10:22 PM, Pars said:

Documentation? heh

Which amp? This is a confusing thread because there are at least 3 amps discussed here, maybe more (Kevin or others, correct me if I am wrong):

1. CFA2 (CFP) - current feedback amp. Each board does 1 channel SE, so 2 for stereo SE, 4 for stereo balanced. Kevin later did a mod to this adding current mirrors feeding the servo. People who had built the original noticed that the servo was audible as to quality of opamp. The schematic in Kevin's archive is the older version.
2. CFA3 - current feedback amp for balanced. Not sure if anyone ran boards for this one. Basically a pair of CFA2 amps and ubal-bal board on one pcb. Schematic does not show the current mirror mod as in 1 above
3. uberamp - seems to be a power amp; I've never quite figured this one out

Here are the schematics for the CFA2 (current mirror version), and the CFA3. I also attached my BOM for a CFA2. Amp quantity shown is for stereo SE. If going balanced, double it. It should reflect the later current mirror version. You'll also need some GRLVs or other power supplies, etc.

cfa2cmirror.pdf

cfa3production.PDF

CFP2_BOM_update2018march.xlsx

EDIT: I also ran across another schematic on Kevin's site which I hadn't seen before: currentsourceamp2.PDF. This seems very similar to the cfa2cmirror schematic, but is described as a transconductance amp.

Your BOM has 1k specified but I can't find that anywhere in the schematic. Is the 1k necessary?

Edit: here's the mouser BoMs for the CFA2 and GRLV I created:

CFA2

GRLV (30v)

I think they're both correct but if there are any mistakes please let me know...

Edited January 20, 2019 by Ariamella

[Pars]

There is a 1K on the v1.10 board. Not on the schematic (hence the R? designation). There is also a 22pf cap on the board, also not on the schematic. These appear to be associated with the input as the 1K runs from the input connector and appears to connect to the 22pf. With Kevin's designs, you always go with the board markings

 

[Ariamella]

Ah, am I blind or is there not actually a 10k on the silk?

[Ariamella]

Also, is matching a must for this amplifier?

[jose]

Match components always helps something. Do it if you can. 

[Ariamella]

Apologies, this is probably a silly question, but what's the most important components to match? (And for the GRLV too...) I'm still a little new to this side of DIYing...

Edited January 20, 2019 by Ariamella

[jose]

Trying to measure all the transistors. On ebay you can find a tester for transistors like this or this...

 

 

 

[Ariamella]

Ah sorry, I mean, which transistors should I match? Is it all of them?