Well, I wasn't planning to cascade them as Papa states in F5Turbo article that even though they are rated at 28V they withstand higher voltage in real life testing and 32V should be ok. At 32V and about 8.5mA I have 272mW dissipation, maximum is 400 so I should be good right?
I could supply 36V to the board and build a cascade point to point near the pins of the input jfets.
The cascade is probably the better solution. It is not only the heat dissipation that might pose a problem but going much over the Voltage limits of the parts. If I want to have extra voltage in the FE to make sure I drive the power stage to clipping then I must aim to the 34-36V region, no way to do that without a cascode. A member recently implemented the F5turbo input stage cascode in the BA3FE pcb, I think I will go that route as well.
So regarding adding a cascode to the BA3FE, another member recently did it using the schematic from the F5turbo, using the same resistor values.
While I understand (I guess) what each resistor does in the cascode section, should I adjust anything for +-36VDC instead of the +-32 from the F5turbo ?
I will build this in a small breadboard and solder directly to the pins of the jfets in the BA3FE.
While I understand (I guess) what each resistor does in the cascode section, should I adjust anything for +-36VDC instead of the +-32 from the F5turbo ?
I will build this in a small breadboard and solder directly to the pins of the jfets in the BA3FE.
Hi Manuel,
There is no reason to omit C1 and C2....they jut give cleaner power to input section.
You should not have any voltage at R12 terminals unless you short output to ground ( effectively transforming R12//R13 into signal load ) or measure end of R12 to ground...which can give you DC output before cap.
Having regulators near is a good thing...look at all Cordell preamps....if you have regulators far from preamp, you should have a cap multiplier right next to preamp PCB.
PS: back at 118 after holidays...call me at 1548 any time
CV
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yes for sure...but i am surprised ZM has not told you to go and build the Pumpkin .....
Done.
I am having trouble nulling the DC offset before the capacitor. The trimpots are quite sensible and can't seem to get then below 10mV. About .75V on the mosfets and they'll burn through the night and I'll measure everything again tomorrow!
After the capacitor I still have offset between 0 and 3mV. Shouldn't it be 0.0mV ?? Or can the capacitor still let some through?
After connecting to my F4 I have 17mV on the output of the F4 so safe to use I guess.
Sounds quite nice, but it is still somewhat "closed" , maybe the C3 Elma Silmics need to settle down and get some electricity flowing through them to get up to shape!
Some pictures:
An externally hosted image should be here but it was not working when we last tested it.
I made a mistake on the back panel and the number order doesn't match the PCB so I had to invert the wires...
An externally hosted image should be here but it was not working when we last tested it.
I made two big rectangular cutouts beneath each regulator heatsink to have a lot of ventilation...
An externally hosted image should be here but it was not working when we last tested it.
... and a vented upper panel.
An externally hosted image should be here but it was not working when we last tested it.
"Loop" is a loop out from the inputs, "0db" is output from the attenuator making it a passive preamp and"gain" is output from the BA3 gain stage. So quite some flexibility!
An externally hosted image should be here but it was not working when we last tested it.
The two female IEC connectors are switched via a 10A/250V relay by the preamp controller so I can connect the phono stage and power amp and make the entire system switch on/off with a single push of a button.
An externally hosted image should be here but it was not working when we last tested it.
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DC offset before the cap will be very difficult to completely eliminate - that's why there is an output cap. Minimum DC offset helps with distortion spectra.
After the cap, 3mV, also written as 0.003V, is basically nothing. Don't worry about it.
Yes, let Silmic caps run in. Also consider a small film in the other C3 position.
After the cap, 3mV, also written as 0.003V, is basically nothing. Don't worry about it.
Yes, let Silmic caps run in. Also consider a small film in the other C3 position.
I will have some better output capacitors, may obbligato/mundorf or audin, but they are quite expensive and will be ordered in a month or so!
One thing I am not getting is enough volume. It plays quite loud but I was expecting a bit more I guess. Speaker are 89dB 6ohm.
I have the attenuator fully open and it doesn't get really loud, just loud. Attenuator Zout is 27k, should I change the termination resistors to alter the ratio and maybe get a bit more volume?
The F4 is running on +-32V and the BA3FE is running at 28V without cascode, will the volume increase when I increase the voltage on the FE with the cascode? It doesn't alter the gain so I guess the answer is no.
I added some pictures in the previous post, check them out!!
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pic2 shows single signal wires without the return current route.
This will make this pre-amplifier more suceptible to picking up interference.
Use a two wire connection for all signals.
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If you look to the middle of the input RCA ground bar you'll see that I have made a central ground connection for all the signals. So GND from theattenuator and preamp pcb meet at a single point and avoid ground loops inside the case.
Wouldn't this be a preferable method?
I have a question regarding gain and preamp psu voltage.
I am currently running the ba3 with +-28V. I will build a cascode to be able to increase the rails to +-34-36V (power amp is +-33 so have a couple extra volts in the preamp is advisable right?). Should I expect an increase it maximum volume with the extra voltage swing in the preamp or is this wrong thinking?
I would like to have a bit more volume range, the first 30 steps in the attenuator are really low and I find myself near the top of the range all the time. Or does this involve changing all the resistors in the stepped attenuator fora different overall impedance?
As you can see I am at a lost here so any help would be great!
I am currently running the ba3 with +-28V. I will build a cascode to be able to increase the rails to +-34-36V (power amp is +-33 so have a couple extra volts in the preamp is advisable right?). Should I expect an increase it maximum volume with the extra voltage swing in the preamp or is this wrong thinking?
I would like to have a bit more volume range, the first 30 steps in the attenuator are really low and I find myself near the top of the range all the time. Or does this involve changing all the resistors in the stepped attenuator fora different overall impedance?
As you can see I am at a lost here so any help would be great!
I don't think so.
But judging by the number of Members who adopt similar BIG LOOP AREA inputs it is very popular and that seems to confirm that they all disagree with me and the low loop area theorem.
I agree. You appear to be lost.
Gain is set inside the pre-amplifier.
Attenuation is set by the position you have set the vol pot to.
eg. Pre-amp gain = +14dB
vol pot attenuation = -8dB
total system gain = +14dB -8dB = +6dB
The output is double the input.
You put in a maximum signal of 2Vac from a CDP and you get out a maximum of 4Vac from the vol pot plus pre-amp.
Is that part understandable?
I understand that!
So in the maximum volume position (in my case, step 64 of the stepped attenuator) there is no attenuation I believe. I get 0.7ohm from signal to ground.
So I need to increase de gain of my preamp then.
The question is: my preamp runs on +-28VDC at the moment and the power amp F4 runs at +-33VDC.
So the preamp doesn't swing enough voltage to make all the use of the voltage swing of the power amp.
Will increasing the voltage swing of the preamp have any direct relation to an increase in db?
So in the maximum volume position (in my case, step 64 of the stepped attenuator) there is no attenuation I believe. I get 0.7ohm from signal to ground.
So I need to increase de gain of my preamp then.
The question is: my preamp runs on +-28VDC at the moment and the power amp F4 runs at +-33VDC.
So the preamp doesn't swing enough voltage to make all the use of the voltage swing of the power amp.
Will increasing the voltage swing of the preamp have any direct relation to an increase in db?
Not really, that is/was my problem.
I've just discovered a post from jump saying that increasing the bias will increase the gain due to increased transconductance.
I've reduced the preamp psu voltage to +-24V and increased the bias to 1V (it was running at 750mV for heat testing) and now it is getting louder indeed.
I get 55C in my infrared thermometer with +-24V rails and 1V across.
I'll order some bigger heatsinks to withstand the higher voltage (+-35/36V) and maybe a bit higher bias and try to bias them as high as needed for maximum volume needed in my application.
I've just discovered a post from jump saying that increasing the bias will increase the gain due to increased transconductance.
I've reduced the preamp psu voltage to +-24V and increased the bias to 1V (it was running at 750mV for heat testing) and now it is getting louder indeed.
I get 55C in my infrared thermometer with +-24V rails and 1V across.
I'll order some bigger heatsinks to withstand the higher voltage (+-35/36V) and maybe a bit higher bias and try to bias them as high as needed for maximum volume needed in my application.