EMI RE-321

Additional Info

From The Tape Recorder, November 1961-
Manufacturer’s Specification: Recorded frequency response: 60 c/s-10 Kc/s to C.C.I.R. recommendation. Signal to noise ratio (at 1,000 c/s unweighted) -44 dB, internal replay facilities with record gain at mid position, and -49 dB when replayed on E.M.I. studio recorder. Wow: better than 0.2% R.M.S. Input: For high impedance microphone 7,000 ohms. Low impedance version : 30 ohms. Output : Phones /line out 600 ohm impedance. Weight 16½ lbs. Batteries 8 1.5 cells. Tape speed : 7½ i/s-15 mins. Price: £124. Manufactured by : E.M I. Electronics Ltd., Hayes, Middlesex.
THIS recorder is a transistorised version of the older Model L2, and is designed for application where extreme portability is required, such as mobile commentating, interview work, etc. No erase head is fitted and bulk erased tape must be used. Separate record and replay heads and amplifiers are provided so that the recorded signal can be monitored directly from the tape a fraction of a second after being recorded. Visual monitoring on the volume indicator meter is recommended for most purpose, as any effort to monitor ones own voice on headphones will quickly lead to a nervous breakdown; the slight time delay between record and play is guaranteed to turn the most hardened commentator into a gibbering idiot within a very few sentences. When switched to replay, the gain to the internal speaker is fixed so that a simple listening test will soon show if the recording is at the right level. Tape overload occurs about 3 dB below that of the output stage feeding the speaker, and any distortion heard is almost certainly a result of over recording. Rewind is manual and a highly geared rewind handle is coupled to the supply reel by depressing a button on the lid.
Wow and flutter: The instantaneous short term speed variations did not exceed 0.1% R.M.S. at any time, even when the tape was played on the portable recorder. This is considerably better than the specification figure of 0.2% max, which is probably only obtained when all tolerances combine to give the worst possible performance. Fig. 1 shows that tape flutter is very low indeed and that a slight wow at capstan rotation frequency of about 10 c/s is just discernable when the record and replay speed variations happen to be in phase. Slow piano music was recorded and listened to most critically and no audible wow could be detected.
Play only response: A full track C.C.I.R. 7½ ips test tape with a surface induction characteristic corresponding to a 100 microsecond time constant was placed on the machine, and the response at the output jack is indicated by the top curve of Fig. 2. This shows that the playback response is within 1 dB from 50 c/s to 10 Kc/s, and that the line level corresponds very closely to the standard 0 dB level of .775v R.M.S. for a 600 ohm line.
Record-play response: The lower solid curve of Fig. 2 shows the combined record-play response, with the test signal injected From a low impedance source in series with a 30 ohm resistor to simulate the output from a low impedance microphone. The input level for 0 dB line level was 50 microvolts with the recording gain control at maximum. The response is within plus or minus 2 dB over the range 100 c/s to 10 Kc/s with a fall of 10 dB at 40 c/s . A further bass cut can be obtained by operating the “bass cut” switch so that a smooth roll off of 12 dB at 100 c/s and 20 dB at 40 c/s is available to compensate for close speech or difficult acoustic environment.
Signal/Noise ratio: At this point we have to throw overboard all preconceived ideas about signal/noise ratios as applied to domestic recorders, where a total range of 40 dB from tape overload to recorded noise and playback hum is considered tolerable. In this machine the transistor noise and induced motor noise is 48 dB below test tape or 0 dB line level, and overload recording tests show that distortion is only 3% at peak recording, or plus 12 dB line level. So that, neglecting tape noise, a potential dynamic range of 60 dB is available.
Further tests with tape running show that bulk erased tape increases the noise by only 1 dB, and that tape recorded on the machine with the recording gain control at minimum is 2 dB up on system noise. Finally, with a dummy source resistor of 30 ohms, and the recording gain full on, the recorded noise, mainly from the first transistor, is 42 dB below 0 dB line level or 54 dB below peak recording level. The use of full track accounts for some of this improvement, but very careful bias oscillator design for low even harmonic distortion and proper balancing of the noise contribution of the various amplifier stages does the rest.
Acoustic response: The response from tape to monitor speaker acoustic output was measured by loading the machine with a White Noise Test Tape and measuring the sound output at one foot from the monitor speaker fret. The response of Fig. 3 indicates that the overall response is within plus or minus 5 dB over the range 150 c/s to 8 Kc/s, This is perfectly adequate for judging voice and background effects balance and, as mentioned earlier, the fixed gain also allows some assessment of recorded level and overload. 0 dB level on the tape at 1 Kc/s gives an acoustic output of 79 phons, or 79 dB above hearing threshold. Peak recording level thus results in a peak sound output of 91 phons. The average domestic recorder acoustic peak output level is about 108 phons so that it will be appreciated that the monitor sound output of this machine is quite low and is only meant to give the operator some idea of the recorded quality and balance when listening about one foot from the speaker.
Comment: As would be expected of a recorder with this pedigree, the design and performance is immaculate and there are a number of interesting circuit design features which are worth noting. For instance; a transistor smoothing circuit is used to isolate the sensitive amplifier circuits from any H.T. disturbances caused by the heavy current drain of the motor. Another transistor isolates the record level meter from the record amplifier output, to avoid any slight distortion caused by the meter rectifier. A push-pull bias oscillator is used to ensure a truly balanced waveform and so reduce recorded noise to the absolute minimum. Three transistors are used in the recording amplifier: a pre-amplifier with slight degeneration in the emitter circuit to cut the extreme bass response, and a two stage amplifier following the gain control with negative feedback from output collector to input emitter, to reduce distortion and to provide pre-emphasis top rise for recording.
Transistors Playback Amplifier
The main playback amplifier consists of four transistors in cascade, with feedback loops around each pair; the first loop modifies the response to give the necessary bass and top boost to correct for tape and head losses, and the second loop reduces distortion in the driver or line output stage and also provides a slight extra bass lift at frequencies below 100 c/s to compensate for deviations from a true C.C.I.R. equalisation characteristic in the first equaliser. The interna1 loudspeaker output stage is brought into operation on playback when the line jack is withdrawn from its socket; it consists of a push-pull pair of small power transistors, working in class AB with a centre-tapped speaker voice coil to eliminate output transformer losses.
I did notice that on record a very faint delayed reproduction is heard on the loudspeaker, even though the H.T. on the power amplifier is disconnected. This is probably due to signals from the driver stage passing through the capacity of the output transistors directly to the speaker voice coil. If the microphone is being used very near to the recorder this sound can be picked up and recorded as a faint echo. The effect is much reduced when the line jack is removed but it is just there, and it is indeed a tribute to the extremely good signal/noise ratio of the recordings that the signal can be heard at all on replay.
Mechanical noise from the capstan drive motor, although barely audible directly, is recorded at very low level if the microphone is brought within a yard or so of the case, particularly if the recorder is standing on a table or surface which can act as a sounding board. When the recorder is slung from the shoulder in the normal interviewing position the radiated acoustic noise is much reduced and is masked by any slight ambient background sounds.
Alternative input circuits for low impedance microphone, medium impedance microphone, or line can be fitted to order, and a rechargeable power pack to replace the 1.5V battery container will be available shortly.
This machine is a studio recorder in miniature, with no relaxing of the specification on the essential recording functions. Only on non-essentials, such as erase, power rewind and monitor power output, have sacrifices been made in the interests of reasonable portability.
Manufacturer’s Specification: Recorded frequency response: 60 c/s-10 Kc/s to C.C.I.R. recommendation. Signal to noise ratio (at 1,000 c/s unweighted) -44 dB, internal replay facilities with record gain at mid position, and -49 dB when replayed on E.M.I. studio recorder. Wow: better than 0.2% R.M.S. Input: For high impedance microphone 7,000 ohms. Low impedance version : 30 ohms. Output : Phones /line out 600 ohm impedance. Weight 16½ lbs. Batteries 8 1.5 cells. Tape speed : 7½ i/s-15 mins. Price: £124. Manufactured by : E.M I. Electronics Ltd., Hayes, Middlesex.
THIS recorder is a transistorised version of the older Model L2, and is designed for application where extreme portability is required, such as mobile commentating, interview work, etc. No erase head is fitted and bulk erased tape must be used. Separate record and replay heads and amplifiers are provided so that the recorded signal can be monitored directly from the tape a fraction of a second after being recorded. Visual monitoring on the volume indicator meter is recommended for most purpose, as any effort to monitor ones own voice on headphones will quickly lead to a nervous breakdown; the slight time delay between record and play is guaranteed to turn the most hardened commentator into a gibbering idiot within a very few sentences. When switched to replay, the gain to the internal speaker is fixed so that a simple listening test will soon show if the recording is at the right level. Tape overload occurs about 3 dB below that of the output stage feeding the speaker, and any distortion heard is almost certainly a result of over recording. Rewind is manual and a highly geared rewind handle is coupled to the supply reel by depressing a button on the lid.
Wow and flutter: The instantaneous short term speed variations did not exceed 0.1% R.M.S. at any time, even when the tape was played on the portable recorder. This is considerably better than the specification figure of 0.2% max, which is probably only obtained when all tolerances combine to give the worst possible performance. Fig. 1 shows that tape flutter is very low indeed and that a slight wow at capstan rotation frequency of about 10 c/s is just discernable when the record and replay speed variations happen to be in phase. Slow piano music was recorded and listened to most critically and no audible wow could be detected.
Play only response: A full track C.C.I.R. 7½ ips test tape with a surface induction characteristic corresponding to a 100 microsecond time constant was placed on the machine, and the response at the output jack is indicated by the top curve of Fig. 2. This shows that the playback response is within 1 dB from 50 c/s to 10 Kc/s, and that the line level corresponds very closely to the standard 0 dB level of .775v R.M.S. for a 600 ohm line.
Record-play response: The lower solid curve of fig. 2 shows the combined record-play response, with the test signal injected From a low impedance source in series with a 30 ohm resistor to simulate the output from a low impedance microphone. The input level for 0 dB line level was 50 microvolts with the recording gain control at maximum. The response is within plus or minus 2 dB over the range 100 c/s to 10 Kc/s with a fall of 10 dB at 40 c/s . A further bass cut can be obtained by operating the “bass cut” switch so that a smooth roll off of 12 dB at 100 c/s and 20 dB at 40 c/s is available to compensate for close speech or difficult acoustic environment.
Signal/Noise ratio: At this point we have to throw overboard all preconceived ideas about signal/noise ratios as applied to domestic recorders, where a total range of 40 dB from tape overload to recorded noise and playback hum is considered tolerable. In this machine the transistor noise and induced motor noise is 48 dB below test tape or 0 dB line level, and overload recording tests show that distortion is only 3% at peak recording, or plus 12 dB line level. So that, neglecting tape noise, a potential dynamic range of 60 dB is available.
Further tests with tape running show that bulk erased tape increases the noise by only 1 dB, and that tape recorded on the machine with the recording gain control at minimum is 2 dB up on system noise. Finally, with a dummy source resistor of 30 ohms, and the recording gain full on, the recorded noise, mainly from the first transistor, is 42 dB below 0 dB line level or 54 dB below peak recording level. The use of full track accounts for some of this improvement, but very careful bias oscillator design for low even harmonic distortion and proper balancing of the noise contribution of the various amplifier stages does the rest.
Acoustic response: The response from tape to monitor speaker acoustic output was measured by loading the machine with a White Noise Test Tape and measuring the sound output at one foot from the monitor speaker fret. The response of fig. 3 indicates that the overall response is within plus or minus 5 dB over the range 150 c/s to 8 Kc/s, This is perfectly adequate for judging voice and background effects balance and, as mentioned earlier, the fixed gain also allows some assessment of recorded level and overload. 0 dB level on the tape at 1 Kc/s gives an acoustic output of 79 phons, or 79 dB above hearing threshold. Peak recording level thus results in a peak sound output of 91 phons. The average domestic recorder acoustic peak output level is about 108 phons so that it will be appreciated that the monitor sound output of this machine is quite low and is only meant to give the operator some idea of the recorded quality and balance when listening about one foot from the speaker.
Comment: As would be expected of a recorder with this pedigree, the design and performance is immaculate and there are a number of interesting circuit design features which are worth noting. For instance; a transistor smoothing circuit is used to isolate the sensitive amplifier circuits from any H.T. disturbances caused by the heavy current drain of the motor. Another transistor isolates the record level meter from the record amplifier output, to avoid any slight distortion caused by the meter rectifier. A push-pull bias oscillator is used to ensure a truly balanced waveform and so reduce recorded noise to the absolute minimum. Three transistors are used in the recording amplifier: a pre-amplifier with slight degeneration in the emitter circuit to cut the extreme bass response, and a two stage amplifier following the gain control with negative feedback from output collector to input emitter, to reduce distortion and to provide pre-emphasis top rise for recording.
Transistors Playback Amplifier
The main playback amplifier consists of four transistors in cascade, with feedback loops around each pair; the first loop modifies the response to give the necessary bass and top boost to correct for tape and head losses, and the second loop reduces distortion in the driver or line output stage and also provides a slight extra bass lift at frequencies below 100 c/s to compensate for deviations from a true C.C.I.R. equalisation characteristic in the first equaliser. The interna1 loudspeaker output stage is brought into operation on playback when the line jack is withdrawn from its socket; it consists of a push-pull pair of small power transistors, working in class AB with a centre-tapped speaker voice coil to eliminate output transformer losses.
I did notice that on record a very faint delayed reproduction is heard on the loudspeaker, even though the H.T. on the power amplifier is disconnected. This is probably due to signals from the driver stage passing through the capacity of the output transistors directly to the speaker voice coil. If the microphone is being used very near to the recorder this sound can be picked up and recorded as a faint echo. The effect is much reduced when the line jack is removed but it is just there, and it is indeed a tribute to the extremely good signal/noise ratio of the recordings that the signal can be heard at all on replay.
Mechanical noise from the capstan drive motor, although barely audible directly, is recorded at very low level if the microphone is brought within a yard or so of the case, particularly if the recorder is standing on a table or surface which can act as a sounding board. When the recorder is slung from the shoulder in the normal interviewing position the radiated acoustic noise is much reduced and is masked by any slight ambient background sounds.
Alternative input circuits for low impedance microphone, medium impedance microphone, or line can be fitted to order, and a rechargeable power pack to replace the 1.5V battery container will be available shortly.
This machine is a studio recorder in miniature, with no relaxing of the specification on the essential recording functions. Only on non-essentials, such as erase, power rewind and monitor power output, have sacrifices been made in the interests of reasonable portability.
A.Tutchings.