How to power a QUAD FM1 ...
- Thread starter Michael J
- Start date
martin clark
pinko bodger
FM1 requires:
+330V @ 30mA
6.3Vdc @ 1.5A
So it's very easy to home-brew-your own. Maplin even used to do a suitable transformer of the shelf. You may need to replace the standard PSU cable/plug for your own 4-core effort though. Its all easy, though: a small 240:240VAC isolation transformer*, bridge rect/ CRC supply for the HT, and traditional AC 6 - 6.3VAC heater transformer. I've tried regulated DC and SMPS and the like for the heaters in the FM1 and it makes no difference over simply biasing the heater supply to about +30V off the HT supply - the traditional answer to valve hum.
Incidentally, the FM1 uses a lot of RC smoothing internally, something like 5kohm/8uF, which if you bypass, can be replaced by about +175-180V regulated DC if that's easier. The tuner will actually function fine down to about +130V DC direct (from experiment) with some reduction in sensitivity; it's essentially '330v'-compatible so it will run straight off the Quad 22Pre supply. Bypassing the internal RC dropper (if you have a suitable raw supply and are happy building an HV reg) means you save about 5w of wasted heat.
*SInce these things are expensive, one version I built uses two small, cheap 240:15 VAC/1A transformers back-to-back. Delivers about 310VDC under load, works just fine.
+330V @ 30mA
6.3Vdc @ 1.5A
So it's very easy to home-brew-your own. Maplin even used to do a suitable transformer of the shelf. You may need to replace the standard PSU cable/plug for your own 4-core effort though. Its all easy, though: a small 240:240VAC isolation transformer*, bridge rect/ CRC supply for the HT, and traditional AC 6 - 6.3VAC heater transformer. I've tried regulated DC and SMPS and the like for the heaters in the FM1 and it makes no difference over simply biasing the heater supply to about +30V off the HT supply - the traditional answer to valve hum.
Incidentally, the FM1 uses a lot of RC smoothing internally, something like 5kohm/8uF, which if you bypass, can be replaced by about +175-180V regulated DC if that's easier. The tuner will actually function fine down to about +130V DC direct (from experiment) with some reduction in sensitivity; it's essentially '330v'-compatible so it will run straight off the Quad 22Pre supply. Bypassing the internal RC dropper (if you have a suitable raw supply and are happy building an HV reg) means you save about 5w of wasted heat.
*SInce these things are expensive, one version I built uses two small, cheap 240:15 VAC/1A transformers back-to-back. Delivers about 310VDC under load, works just fine.
martin clark
pinko bodger
That'll probably drop a bit under load - the heaters in an FM1 draw about 1.5-1.8A.
If still high, just add a small power resistor in series with it. 0.33ohms - three 1R resistors in parallel - would knock about 0.5-0.6v off that under load - perfect
They'd disspate about a watt in total, i.e. run rather hot.
If still high, just add a small power resistor in series with it. 0.33ohms - three 1R resistors in parallel - would knock about 0.5-0.6v off that under load - perfect
They'd disspate about a watt in total, i.e. run rather hot.