Croft Charisma Pre-Amp

 

 

 This modern pre-amplifier arrived in March 2025 for an "overhaul". Although people tend to be quite excited by it I found it was a bit amateurish in build quality. Then again, I think the same of Roberts radios which are pretty clever designwise. This example hasn't got the build quality that one might expect from going market prices because all solder joints are of the "just held and touch soldered" style, although testing a component takes only seconds because of this. Wiring is neat but wire diameters pretty small and many wires running close together are uninsulated.

 

 

 Valves are unusual, having "high quality" coding (even the voltage regulator). Resistors are a mixture of exceedingly high tolerance types, wirewound ceramic and standard 4-ring types. Presumably these relatively expensive parts are used to keep overall operating tolerances to a minimum although drift from design figures due to valve ageing might make this aspect a waste of time and money (static voltage measurements proved this to be indeed the case).

A circuit diagram is hard to find so I've drawn one up from my examination of the thing, below. Oddly there are a few design anomalies. the heater supply is a regulated 12 volts which measured 11.94 volts in this example when the filament voltage should ideally be 12.6 volts. The addition of an extra diode or two and this could be fixed. Another mistake is to have missed a decoupling capacitor at the DC heater supply. In addition, although very high tolerance resistors are used, there are a few examples of +/- 5% coupling capacitors, both 100nF and 1uF type MKP1841 which defeats the object when it comes to precise audio balance.
 

 

 Below is the power supply schematic. The rectified HT can be a trifle high considering the regulated voltage level of only 85 volts. A separate stabiliser heater voltage is used because of the elevated cathode voltage at V7a. Strictly speaking, no capacitor is required at the output of the LM340T12 but surprisingly I found no decouplers in the heater circuit at all.
 

 

 The pre-amp seems to be OK electrically. The only static powered test I did was to measure the grid voltages at V1b, V3b, V4a and V4b. I found V1b was sitting at greater than 125mV whilst the others were close to zero. I had a high quality "7025A" which is identical to the 12AX7 except it uses a "long" anode against the 12AX7WAs "short" anode so, in the interests of balancing audio, I swapped the new 7025A with V4 (for the reasoning see the way the triodes are arranged in the circuit diagram above) and put V4 in the place of V1.

I'll be testing audio performance in a day or two. Scroll down to see the results.
 

 

 A few comments on this pre-amplifier. When I started looking around the Net for details on it I became absorbed in the comments. It seems it's not a static design or that it suddenly appeared in the form above. For example this one uses (in its audio section at least) two types of valve viz. 12AU7WA and 12AX7WA. These are relatively new versions of the 12AU7 (1951) and 12AX7 (1954) which are basically the same as the "British/European" coded ECC82 and ECC83. If we go back in time before those B9A all-glass based valves were introduced the equivalents of theSE were the 6SN7GT (1941) and 6SL7GT (1942), the short tubular style or without the "GT" being the bulbous style envelopes. The 6SL7 in turn was a development the earlier 6SC7. One distinguishing difference between the various types is the heater voltage/current so for example the newer ones can use either a 6 volt or a 12 volt power supply (Pin 9 is a heater centre tap... which isn't used in this equipment.. but allowing for a 6 volt supply). The newer of the valves mentioned are designed to be run from either 6.3 volts 300mA or 12.6 volts 150mA. This can be DC or RMS AC. Generally speaking the emission of a valve cathode reduces with use and if the heater is run at a reduced voltage then ageing will be accelerated.

I guess the important question is whether or not the choice of valve has any effect on audio fidelity? Well, it certainly does, but many other things come into play also. Ideally whatever sound is listened to should have been recorded and subsequently reproduced to give one a precise copy of the original sound. When one listens to a sound and turns up or turns down the level it should get louder or quieter but should have the same characteristics as the original. The end result will depend on loads of things but I'm afraid the valves used in a pre-amplifier will not figure much if they're doing a proper job. I guess, for a pre-amplifier such as this the most noticeable weakness will be hum or sound distortion. You'll be able to find examples of the "Charisma" pre-amp using the earlier valves I mentioned above but why were these used exactly? You'll also find comments about the make of volume controls and strange comments regarding differences in sound quality. My comment on this sort of thing is that the volume of left and right sound channels can be independently twiddled so why bother with careful matching and high tolerance resistors?

Finally, if true sound reproduction is really important, then forget about FM radio which is a mish-mash of processing, CDs which use bandwidth restricted digital data, cassette tapes which are subject to recording media failings and Dolby processing and vinyl records which rely on mechanical interaction between a needle and bumpy plastic.

 

 Test results:

Tests were pretty basic with an audio generator driving L & R inputs in parallel with a twin channel scope at either L or R output and the test input. I was using a direct coax lead with no attenuator on the signal generator for Blue Channel 2. For Yellow amplifier output Channel 1 used a 10dB probe which catered for the voltage readings below.

An audio level of 1 volt is considered to be 0dBV, with a domestic standard of -10dBV representing 316mV.

A VU meter is generally said to read "0" for 775mV RMS so tests were carried out using voltages slightly in excess of VU zero.

Both L & R volume controls set at max for all tests

Despite its physical size and overall complexity the amplifier is using only a couple of triodes for amplification of Tuner, CD and Tape with an extra three triodes for additional amplification of a gramophone pickup.

 

 

 Tuner input 1Hz Left output

Gain 1.38/0.347=4

 Tuner input 1Hz Right output

Gain 1.39/0.346=4

 

 

Tuner input 25Hz Left output

Gain 5.55/1.31=4.2

Tuner input 25Hz Right output

Gain 5.52/1.31=4.2

 

 

Tuner input 1KHz Left output

Gain 5.68/1.34=4.2

 Tuner input 10KHz Right output

Gain 6.03/1.35=4.5

 

 

 Tuner input 20KHz Left output

Gain 6.04/1.35=4.5

Tuner input 50KHz Right output

Gain 5.80/1.35=4.3

 

 

Tuner input 100KHz Right output

Gain 4.92/1.31=3.8

Tuner input 200KHz Left output

Gain 3.29/1.25=2.6

 

 

 Tuner input 1KHz Right output max just before clipping

Gain 22.6/5.44=4.2

 Pickup input 1KHz Right

Gain 39.6/.0795=498

 

 

Tuner input open. Left output

Unterminated input socket residual 50Hz hum of 65mV.

Note that HT hum would be 100Hz so this reading is from stray earth currents in the test environment plus possible microphony in V4/V5 valve electrodes from the 50Hz mains transformer.

 Tuner input open. Right output

Unterminated input socket residual 50Hz hum of 46mV.
 

 

 All the tests were carried out using a single test tone at various frequencies and levels. I didn't attempt to carry out tests using simultaneous test tones or to check linearity or harmonics. Neither did I attempt to listen to the results, merely relying on oscilloscope traces. I didn't bother to simultaneously compare the left and right channels because there are twin volume controls making that exercise irrelevant. Finally, although improvements could have been made (eg the valve heater circuitry), I left the equipment as originally manufactured.

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