This preamp was unusual in that it was a battery powered using 2 PP9 dry batteries. These lasted around 6 months per set. The nominal output level was 775mV RMS but the output stage did not clip until greater that 4V RMS. The inputs were tuner, disc (vinyl) and tape. The two line inputs were sensitive giving 1V RMS output for 67mV input. The disc input required 4.4mV for full output. There were add-on PCBs for moving coil cartridges which gave an extra x30 of flat gain. The line stage had an integral high pass filter at 20Hz -3dB. The line stage had jFet FETs to mute the signal preventing on/off thumps in the speakers. The main PCB used hybrid bipolar transistor/op-amp gain stages.
Performance and Specifications
The disc stage was quiet in operation and outperformed a lot of other preamps at the time by having a very high feedback and therefore accurate RIAA response. Line stages also had a good signal to noise ratio. Hum levels were near zero as long as the unit was well away from power transformers. The sound was accurate but a little dull. The bass end was too light on all inputs. The volume control sometimes dropped one channel at low levels. The silver plated selector switches contacts would tarnish in a few years and could crackle and become intermittent. Batteries generally lasted 6 months or more. The j-Fet mute was easily damaged by earth leakage from two wire equipment and one of both channels would remain muted and/or distorted.
Gain Disc x 17 (1kHz) (25dB) 5,5 mV for 1V RMS output (volume set full)
Line stage gain x 10.75 (21dB) This is 93 mV for 1V RMS output.(volume set full)
510 Updates
- Replace electrolytic capacitors on the main PCB
- Replace the slow LM301 op-amps with high speed types e.g. AD825.
- Wire out the high pass filter. This really brings the bass end up to full strength
- Add selected components to get the RIAA disc equalisation closer to ideal.
- Add ESD protection diodes to prevent jFet mute damage.
- Replace the selector switch with gold flashed types.
- Replace the 2 batteries with holder packs to take 6AA in each. Expect 60 hours run time per set. Alternatively fit a set of 6+6 NiMh rechargeables and use a 50mA trickle charger. Alternatively fit a mains PSU via an adaptor.
- Add on a battery low indicator to make the green LED go red on low battery.
- Replace all connectors with gold flashed types (apart from the earth post)
- Replace the volume potentiometer with better one that tracks well to very low levels.
- Replace the (unused) charger jack with headphone socket (depending on the IC used see above)
- Drill the rear chassis and add 2 extra phonos as line inputs either side of the earth post. The later selector switch PCB can accept up to 6 line inputs. This extra pair will bring the line inputs up to 3 instead of 2 which will be referred to as “aux”. Alternatively, add a 3.5mm stereo jack as a spare line input. This is neat and easier to do.
- As the gain is rather high for many of today’s signal sources, it is a good idea to fit passive attenuators at the inputs phonos so that there will be a) one tape input on the top switch unchanged at 93mV sensitivity b) one tape input (lower switch position 3) {now aux}) at 660 mV sensitivity{22k/3k9} c) Tuner input at 300mV sensitivity{22k/10k}. Alternatively modify the board for less line stage gain and increase the disc gain to suit.
The 510 had a rather irrational input switching system. There were two line inputs ,Tuner and Tape. There was concern that when a tape recording was being played backed that the tape out would cause a regenerative signal and oscillate. The lower switch was used for normal tape playback. The top switch was used to listen in on the after record head (3 head machine) playback. When the switch was used the tape out signal was muted to prevent feedback. When the upper switch was used the disc or tuner signal carried on to the tape machine so as not to interrupt the recording. Nowadays hardly anyone uses tape. So this input system and monitor is not required. So the best use for the switching is to have the top switch tape position labelled “Mon” as a general purpose line input. The lower tape position can then be the “aux”. This creates an extra line input without any changes to the input panel at the front. The lower switch “tape” becomes the “aux” input.