As this is an advanced project and its obvious that you are not experienced in amp building, you will need assistance with this build.
Before you buy a tranformer(s) or switch mode power supply we need to determine which output transistors you will need.
Is your 4 ohm speaker difficult to drive? Have you got a impedance plot of this speaker?
Your answer will help us determine the rail voltage for your transformer, so you do not exceed the SOA of the output transistors.
Before you buy a tranformer(s) or switch mode power supply we need to determine which output transistors you will need.
Is your 4 ohm speaker difficult to drive? Have you got a impedance plot of this speaker?
Your answer will help us determine the rail voltage for your transformer, so you do not exceed the SOA of the output transistors.
The single secondary with CT uses only one bridge rectifier. Therefore, you get additional output voltage on each rail of one diode drop, since each rail has it's own conducting diode. This comes at the cost of potentially coupling some of the secondary noise into the DC ground. Also , the bridge must be rated for twice the voltage, since the sum of the secondary voltages appear across the diodes. There is also a possibility of some DC causing noise from the transformer.
With the separate secondaries and two bridges you get an additional diode drop in the rail circuit on each side since two diodes are in the conducting path for each separate rail. That lowers the rail voltage around .7 - 1 volt under load. The benefit is that you can avoid tying the secondaries into the DC ground, and the diodes can be half the voltage since each diode sees only the peak to peak voltage of a single secondary.
With the separate secondaries and two bridges you get an additional diode drop in the rail circuit on each side since two diodes are in the conducting path for each separate rail. That lowers the rail voltage around .7 - 1 volt under load. The benefit is that you can avoid tying the secondaries into the DC ground, and the diodes can be half the voltage since each diode sees only the peak to peak voltage of a single secondary.
All true Harry. It's a challenge indeed but I WILL get there🙂. I do have the dats 3 to measure my speakers (kef reference model three). I can provide the impedance plot of the whole speaker. Not the the individual drivers as I dont want to unmont the drivers yet🙂.
You don't need to measure it. Have you got a brochure that has the impedance plot? I want to know if it dips below 3 ohms.
I found this on the net regarding your speakers:
"With its high sensitivity of 91dB for 2.83V at 1m and honest nominal load impedance of 4 ohms (I measured minimum values of 3.5 ohms or greater at 63Hz, 315Hz and 16kHz) this loudspeaker is suitable for amplifiers rated at 50W and above."
I'll come back to you when I get some time.
"With its high sensitivity of 91dB for 2.83V at 1m and honest nominal load impedance of 4 ohms (I measured minimum values of 3.5 ohms or greater at 63Hz, 315Hz and 16kHz) this loudspeaker is suitable for amplifiers rated at 50W and above."
I'll come back to you when I get some time.
Im afraid not. These speakers are from the nineties. I asked Kef UK for the driver plots but they couldn't provide it. I did measure the speaker a couple of weeks ago and from what I recall it did drop slightly below 3 ohms in parts of the lower area (< 1khz) iirc
https://www.hifiengine.com/manual_library/kef/reference-model-three.shtml
https://www.hifiengine.com/manual_library/kef/reference-model-three.shtml
Are these your speakers?
https://www.gramophone.co.uk/features/article/review-kef-reference-model-three-loudspeaker
https://www.gramophone.co.uk/features/article/review-kef-reference-model-three-loudspeaker
This is what I would choose for 3 output pairs, as your speakers go below 4 ohms.
MJL3281 and MJL1302A:- Max 60Volt DC Rails (42V AC)
MJL4281 and MJL4302A:- Max 64V DC Rails (45V AC)
Examples of transformers below:
I’ll let someone else help you with the heatsink requirements and transistor placement as I’m looking after my son who has covid. If I remember correctly the MJL4281 and MJL4302A need drilling to fit on the heatsink.
Edit: I would use 42 Volt AC or less transformers for your build.
2nd Edit: You can use a SMPS with +-60V dc.
MJL3281 and MJL1302A:- Max 60Volt DC Rails (42V AC)
MJL4281 and MJL4302A:- Max 64V DC Rails (45V AC)
Examples of transformers below:
- One transformer at 800 or 1000VA with 4 windings (as Vunce pointed out above) e.g. 4 x 42VAC windings.
- Two transformers at 500VA each with 2 x 42V AC windings on each transformer.
I’ll let someone else help you with the heatsink requirements and transistor placement as I’m looking after my son who has covid. If I remember correctly the MJL4281 and MJL4302A need drilling to fit on the heatsink.
Edit: I would use 42 Volt AC or less transformers for your build.
2nd Edit: You can use a SMPS with +-60V dc.
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Thx Harry...appreciate your help and advise. I'm in the collecting phase and this certainly helps. We all had covid overhere and it can be nasty if you are unlucky.
reg
willem
reg
willem
Could someone give me a simulated schematic in Micro-Cap 12. I need to reduce the gain of the amp to feed the driver with 110dB sensitivity. This will necessarily lead to adjustment of compensations. I want to fit a low power version of the EF3-3 with one or two pairs of MJL1302/3281.
Thanks, Lubo
Thanks, Lubo
You will have to create you own sim cct, no one has M-C 12
But why not reduce the gain with an input voltage divider.
But why not reduce the gain with an input voltage divider.
Morning Gents,....any advise on the chassis choice? Was looking in the diy store and noticed the input RCA holes/mounting in the back plate of the DELuxe 4U have a big distance between them. The wolverine wiring manual stating they have to be as close together as possible. anyways: looking for some chassis advice in general.
thx
thx
You are in Europe, so you can order the chassis directly @modushop from Gianluca. In this way you can customize the holes pattern in the way you like. You could PM him and ask for details.
Gaetano.
Gaetano.