21 hours ago, audiostar said:

Could you please check out that XLR cable you connect your DAC and amp, how is it wired internally? Pin 2-2 and 3-3 are ok, question is about pin 1: does it connect to the shells on both sides as well or is it pin 1-1 only and does it connect pin 1-1 on both sides of the cable?

Hi @audiostar, that cable is pin 1-1 only. But I think it is not that problem, because this noise is there when inputs are floating too with no device connected.

21 hours ago, audiostar said:

Strange as well, have you tried with another pot?

I've no other 4 gang pot here, but I've tried with Kevins stepped digital attenuator and it was the same unf.

 

21 hours ago, audiostar said:

As you basically have a dual mono setup, try completely separating ground for both channels, and connect then each ground centrally to the chassis (via the 10R resistor as you did or a real circuit breaker). So you will have a star ground per channel which is connected to another star (chassis at IEC connector with both channels master grounds).
What you did with separating ground on the pot was the right beginning in this direction.

Thank you, I've not tried this option so far I'll try it.

19 hours ago, MLA said:

I would suggest go full dual mono as you've started above, with separate pot ground for left and right channels.

Hi @MLAthank you for ideas.  This is how I have it now. Two pot gangs are connected to one channel including separate GND to CFA PCB and another two to the other side.

 

19 hours ago, MLA said:

Instead, try connecting the left amp board ground directly to the left GRLV ground and the right amp board ground directly to right GRLV ground, and then reference only the GRLV grounds and pot grounds (not amp grounds) to chassi ground, either directly or through 10R.

This is how it was done first when I've setup this amp for the first time and before I wanted to do it the "right way". I've reverted to this now, but it didn't helped 10R to chassis or not doesn't make difference.

19 hours ago, MLA said:

Since there is bias on the inputs of the CFA3 but it's also a balanced amp, an additional thing to try is disconnecting pot ground altogether. It should in practice be happy with a differential input, as long as dac and amp chassis have a good grounds connection.

I was not aware that it can work this way too, will try... thank you!

18 hours ago, audiostar said:

Yes, very good advice! This is exactly what I have in my two chassis CFA3 with external dual PSU. Two umbilicals carry the separate GRLV grounds to the amp boards directly. Everything connects together in the PSU case including the chassis and IEC earth.

I've adapted to this now: GND and power + and - goes from GRLV to each CFA3 PCB for each channel. Another wire goes from each GRLVs GND to central GND, wago used in my case here.

No difference at all here unf.

45 minutes ago, migo said:

No difference at all here unf.

Yes, this was expected. The different ground schemes discussed here are more like fine tuning and personal preference. All of those are good and you did it already good. Must be something else hidden somewhere; are you sure you are not touching ground from some of the PCBs with the chassis, like when using metal screws or the like to mount the PCBs in the chassis? 

1. Just for the sake of experiment, what happens if you put all PCBs on the table without a chassis and connection to it? I am talking everything out not just disconnecting to it.

2. What happens if you completely eliminate the pot and measure again?

 

Edited October 15, 20223 yr by audiostar

1 hour ago, migo said:

Hi @audiostar, that cable is pin 1-1 only. But I think it is not that problem, because this noise is there when inputs are floating too with no device connected.

If the source was connected I would still suggest to cut pin 1 at the receiver side. You don't know how the source is implemented and where is pin 1 connected to. It may be chassis, may be circuit ground, maybe nowhere.

But it looks like you don't have a source connected at all. What's then the purpose of that XLR cable, don't understand?

14 hours ago, audiostar said:

But it looks like you don't have a source connected at all. What's then the purpose of that XLR cable, don't understand?

Sorry for confusion the cable has source connected, it is standard cable. With source disconnected I mean that my CFA3 amp has this noise even with no source connected to it = with floating input.

I would put all PCBs on the table without a chassis and see what happens.

Second, completely eliminate the pot and measure again.

Make it as simple as possible.

I have a query in regards to the CFA 2 balanced amp (4 boards build).

Will this be suitable to drive low impedance loads around 23 ohms with good current?

Thinking about using for a Dan Clark Audio planar.

I appreciate any help.

18 minutes ago, Roger said:

I have a query in regards to the CFA 2 balanced amp (4 boards build).

Will this be suitable to drive low impedance loads around 23 ohms with good current?

Thinking about using for a Dan Clark Audio planar.

I appreciate any help.

Yes, it will be.

Both CFA2 and CFA3.

Really appreciate your help!

Some folks “over there” seem interested in Kevin’s Tube Hybrid. So, I removed a lot of small components from CFA3smd boards and replaced them with dual triode and double op amp. All according to info I found in Kevin’s library on google drive.

It should all fit in the same box as CFA3smd. PSU needs additional voltages, 5- and 100-volt sections and filament for tubes.

Below is Tube Hybrid schematic. Why op amp U3?

needed to invert the signal to the integrator. a non-inverting integrator did not have enough voltage gain.

thanks

I’ve build a balanced CFA 2, 4 boards balanced in and out. Using with a DCA Audio Expanse planar. 
The amp drives these fine although volume is around about 3.00 o clock on most material, I still have some volume left even on the quietest tracks.

Is it advisable to increase gain, or leave as is? Is there any benefit to increasing?

thanks,

 

On 11/3/2022 at 4:39 AM, JoaMat said:

Some folks “over there” seem interested in Kevin’s Tube Hybrid.

I'm NOT some folks!

Stellar job, btw. My main beef with the OG board was that it's too narrow to fit in a dissipante case without some chunky brackets. I sold off my CFA3 because I never used BAL outputs, so I'm exploring ways of building something SE and smaller footprint wise.

that board was designed for a specific chassis. which one, i no longer remember. i could make it fit a current chassis if that is what people want.

41 minutes ago, kevin gilmore said:

that board was designed for a specific chassis. which one, i no longer remember. i could make it fit a current chassis if that is what people want.

Can't tell what people want. I didn't expect my CFA3 thread to blow up like that. As I said, I'm pretty pessimistic about DIY these days. Don't want to go all grumpy old guy that younglings can't be bothered to build stuff, so I'd actually "blame" there generally good selection of equipment that's available for buying these days.

For the original th2 + thps combo this enclosure seems like the best option. 250x300mm inside space means that around 190mm of length is used for PCB's an the rest 110mm can be taken by connectors and the trafo.

Of course, then there's the @JoaMat PCB which should fit on the 2U heatsink saving up even more space on the "floor" of the enclosure. If there's a matching board for the PSU (say, the thps) which would fit the other 2U heatsink, then something like this (or a longer enclosure) could be used, if SE is all one wants.

Kevin's split boards fit the heat sinks of a 2U case perfectly as well.
All the inner space is then left for whatever one wants. 

On 9/28/2022 at 2:51 PM, JoaMat said:

My CFA3smd something,

now powered from a switched mode power supply and equipped with a headphone protector. I’ll test this for a while and if I like it I might build a decent cage for it. Else I’ve a growing "good to have" closet.

Now I’ve listened to CFA3smd something with switched mode power supply for almost 2 months and I’m happy with it. 

So, no plan for a golden reference supply. I’m too lazy and I think the switched power supply is good enough for me.

Next “project” is how to power a Tube Hybrid. The Hybrid needs +100V, -5V and 6.3VAC for the tubes. The PSU has to be rebuilt. The amplifier board is finished, in theory…

a pre turkey day new year teaser.

sanken output transistors, in parallel.

with appropriate switching power supplies, capable of about 25 amperes.

can be built with 30v, 35v, 40v power supplies

if you change out the pzta06/pzta56 with bf722/bf723 then you can go up to 60v supplies.

with 40v supplies and big enough heatsinks, 100 watts rms into 8 ohms.

lets see how much joamat can shrink this one.

balanced output impedance, .5 ohm

 

 

Edited November 20, 20223 yr by kevin gilmore

Hey Kevin,

thanks for this - specs look like of a power amp 🙂 wow. Gerbers available as well?

gerbers will be available after much more checking. lots and lots of editing was done.

but if you really want to be first, i can arrange for gerbers to be delivered. you can do the checking before making boards.

i got to hear a cfa3 driving vintage klipsch belle.  stunning.  also it was running stupid hot. so this will take care of all of that. should make all the raal owners real happy. (and deaf) (and fried diaphrams)

connex switching power supplies recommended.

a linear power supply would be very large and heavy.

Sure, I can do the checking.

Which Connex would you recommend?

Will the circuit PSRR be enough for SMPSU usage? I'm generally pretty chill about SMPSU's as they often eliminate any kind of hum. At the same time this is a very fast amp and I would like to avoid HF noise on the outputs or even self-oscillation.