Hello dear forum members,
I'm a designing a commercial preamplifier with tape recorder support. It will contain:
So my question is mostly about the tape recorder input level and gain which will be applied to the signal before the output. Is there any right assumption? Perhaps any standard on what a recorder should do to the levels on the input and outputs?
Quick investigation shows that my Revox B77 is applying about 32.3 dB of amplification, when input level knobs are fully open, meaning its input is expecting about 18.8 mV for its nominal output of 0.775 dBu. Having signals of tens of millivolts is rather weird, when most of the todays consumer equipment is having output levels of volts.
PS: My internal preamp nominal level is 150mV.
Will appreciate your suggestions, and feel free to suggest the right forum category.
UPDATE: Correction: the B77 nominal output is 0.775 Vrms, 0 dBu, not 0.775 dBu.
I'm a designing a commercial preamplifier with tape recorder support. It will contain:
- dedicated tape input,
- dedicated tape output,
- "monitor" switch allowing to route main output signal from tape input to the tape output
So my question is mostly about the tape recorder input level and gain which will be applied to the signal before the output. Is there any right assumption? Perhaps any standard on what a recorder should do to the levels on the input and outputs?
Quick investigation shows that my Revox B77 is applying about 32.3 dB of amplification, when input level knobs are fully open, meaning its input is expecting about 18.8 mV for its nominal output of 0.775 dBu. Having signals of tens of millivolts is rather weird, when most of the todays consumer equipment is having output levels of volts.
PS: My internal preamp nominal level is 150mV.
Will appreciate your suggestions, and feel free to suggest the right forum category.
UPDATE: Correction: the B77 nominal output is 0.775 Vrms, 0 dBu, not 0.775 dBu.
Last edited:
Not weird at all.
Golden era preamps had tape head and phono inputs sharing the same circuitry, just switching the eq.
Golden era preamps had tape head and phono inputs sharing the same circuitry, just switching the eq.
I believe the first answer is not according to the question.
Tape monitor switches in amps and preamps are always without any level change or means to adapt the level.
The tape machine itself has to maintain level accuracy between record and playback.
Have a look at standard schematics, tape monitor is just another line input switched in a different way.
Record outputs have the same level as the line inputs they are derived from.
Your third point ""monitor" switch allowing.. " seems to indicate you do not understand preamp wiring.
Tape monitor switches in amps and preamps are always without any level change or means to adapt the level.
The tape machine itself has to maintain level accuracy between record and playback.
Have a look at standard schematics, tape monitor is just another line input switched in a different way.
Record outputs have the same level as the line inputs they are derived from.
Your third point ""monitor" switch allowing.. " seems to indicate you do not understand preamp wiring.
Last edited:
You may be conflating a tape head input, meaning an input built to accept a signal straight from a magnetic tape head, with a tape line level input, meaning built to accept a signal from the complete tape recorder's electronics that has already been equalized and amplified to line level.
A preamp's "tape monitor" cannot work in the first case, but is for the second case. It is located immediately downstream from the input selector switch.
All good fortune,
Chris
Or, maybe this is the source of confusion. Input sensitivity is (must be) always greater than any expected input signal. Excess signal is just burned off by by the input level controls. Your B77 also has output level controls allowing you to match volume levels when comparing source vs. recorded with your tape monitor switch.
ps: 0.775VAC = 0dBu
All good fortune,
Chris
Sorry. It just means mixing two related ideas and assigning conclusions true to idea A to idea B. I think a lot of confusion was caused by using the term "dedicated tape input", which, to folk familiar to hifi history would mean an input designed for direct connection from a magnetic tape head. These were common in early (1950s) days for use with playback-only tape decks, often with no electronics - all mechanical, like a conventional phono deck still is.
All probably just compounded misunderstandings, because language is tricky, and typing is a ridiculous form of communication.
All good fortune,
Chris
All probably just compounded misunderstandings, because language is tricky, and typing is a ridiculous form of communication.
All good fortune,
Chris
Ah, this is not in my dictionary.
It is almost sure that OP wants to use contemporary machines (like Revox B77).
It is almost sure that OP wants to use contemporary machines (like Revox B77).
If you do want to input the signal DIRECTLY from a tape HEAD, you will need around 50dB of gain. And have the ability to "de-equalize" for pre-recorded equalizations, most notably NAB and IEC. There is a lot of tape preamp information on this "out there"
If you are accepting the already amplified and de-equalized signal from the OUTPUT of the tape deck, Your preamp sensitivity is OK but you may want to put an input level potentiometer for the tape deck input to be able to "turn down" the signal if its too great.
If you are accepting the already amplified and de-equalized signal from the OUTPUT of the tape deck, Your preamp sensitivity is OK but you may want to put an input level potentiometer for the tape deck input to be able to "turn down" the signal if its too great.
Last edited:
And FWIW, some preamps have two input selectors, one for recording and one for listening.
With that you can record from one source while listening to another.
In this case you don't need a tape monitor switch, you just select tape on the listening input selector.
All done with simple line level switching at the front end and no level adjustment within the preamp.
With that you can record from one source while listening to another.
In this case you don't need a tape monitor switch, you just select tape on the listening input selector.
All done with simple line level switching at the front end and no level adjustment within the preamp.
Useful advice, but for some the second full (rotary) switch is too expensive.
One advantage of the monitor switch is you can flip between input and tape
to compare recording quality.
One advantage of the monitor switch is you can flip between input and tape
to compare recording quality.
I initially thought you were talking about designing a tape pre-amplifier. (I have done two tape playback amp designs in the past few years for NAB/IEC 7.5ips/15ips, etc.)
I think what you are really talking about is a stereo pre-amplifier design with support for a tape recorder, with tape out, tape in, and monitor provisions. For tape in you might want adjustable gain (switch or jumper) of as much as 12dB to allow tape playback levels to match redbook (2Vrms) which has become the standard for line level in recent decades.
Tape out probably does not need to be attenuated, but you could provide the option. IMO the output should be buffered so that the recorder input circuitry does not load down the pre-amplifier's internal circuitry.
Tape monitor switch can be placed after the source selector.
Suitable rotary switches for source selection have become expensive. You can also consider logic controlled relays, but select such relays carefully for operation at low signal levels.
I run Studer, Otari, and Sony recorders here at home and find monitoring off tape during recording to be a useful feature.
I think what you are really talking about is a stereo pre-amplifier design with support for a tape recorder, with tape out, tape in, and monitor provisions. For tape in you might want adjustable gain (switch or jumper) of as much as 12dB to allow tape playback levels to match redbook (2Vrms) which has become the standard for line level in recent decades.
Tape out probably does not need to be attenuated, but you could provide the option. IMO the output should be buffered so that the recorder input circuitry does not load down the pre-amplifier's internal circuitry.
Tape monitor switch can be placed after the source selector.
Suitable rotary switches for source selection have become expensive. You can also consider logic controlled relays, but select such relays carefully for operation at low signal levels.
I run Studer, Otari, and Sony recorders here at home and find monitoring off tape during recording to be a useful feature.
For an external reproduction amplifier, the capacity of the connecting wires can become a problem. The magnetic head has a high inductance, often the input circuit forms a LC-circuit, set up at the upper operating frequency. This is used to compensate for losses. For each magnetic head, the capacity must be its own. The cable capacity can be too high that it will shift the resonance in the sound range of frequencies.
Another problem can be interference with a cable. The signal level from the magnetic head is compared with a level for MC-cartridges, but the output impedance of the magnetic head is much higher.
For these reasons, the reproduction amplifier is trying to place next to the magnetic heads.
Another problem can be interference with a cable. The signal level from the magnetic head is compared with a level for MC-cartridges, but the output impedance of the magnetic head is much higher.
For these reasons, the reproduction amplifier is trying to place next to the magnetic heads.
Depends on the heads. Been awhile since I played with tape.
50 dB sounds about right.
As similar with phono can range 40 to 60 dB
As mentioned is a good feature almost needed to have a little actuation.
6 to 15 dB
depends
assuming .775 or more modern 1.5 to 2 volts
50 dB sounds about right.
As similar with phono can range 40 to 60 dB
As mentioned is a good feature almost needed to have a little actuation.
6 to 15 dB
depends
assuming .775 or more modern 1.5 to 2 volts
Not sure if it's necessary to say, but some discussion here (and lots elsewhere) seems to conflate (seems a popular word today) signal level standards from analog media and those from digital, which come from different origins and aren't at all equal.
In analog era storage media signal levels were referenced to "0VU", a practical, working-man's, pragmatic estimate of average level, well defined for rise and fall times (meter ballistics) but expecting some (undefined) peak capability beyond 0dBVU. The best professional media, both tape and vinyl, might (very rarely) approach +30VU, and +20VU overhead capacity was expected for the best commercially available recordings. But these levels were always referenced to some physical property of the media. For tape, 0VU would be some level of magnetization, like 185nW/m or something similar, and for vinyl phono it settled on a value of 5cm/s (although even that could vary between meaning horizontal or orthogonal, 3dB different). This would represent the average value of a fairly loud passage, and would expect larger instantaneous peaks.
Digital is different, because it has a hard upper limit, so is referenced to the peak (and limiting) value. If digital had a "0VU" it would be somewhere well below rated (reference) level, to allow for peaks. For example, a digital recording with 20dB of headroom allowed above "average" (VU meter ballistics weighted) through a conventional CD player rated for 2VRMS output will have the same "0VU" signal playback level (200mV) as a vinyl disc played through electronics of sensitivity 200mV for 5cm/s or tape head electronics of 200mV for 185nW/m (or some number close to that - it varied a little).
So digital players are rated at their peak possible output, rather than some averaged signal value, and only indirectly comparable. Everything before the volume control needs to accommodate these levels; everything after the volume control lives in its own world.
All good fortune,
Chris
In analog era storage media signal levels were referenced to "0VU", a practical, working-man's, pragmatic estimate of average level, well defined for rise and fall times (meter ballistics) but expecting some (undefined) peak capability beyond 0dBVU. The best professional media, both tape and vinyl, might (very rarely) approach +30VU, and +20VU overhead capacity was expected for the best commercially available recordings. But these levels were always referenced to some physical property of the media. For tape, 0VU would be some level of magnetization, like 185nW/m or something similar, and for vinyl phono it settled on a value of 5cm/s (although even that could vary between meaning horizontal or orthogonal, 3dB different). This would represent the average value of a fairly loud passage, and would expect larger instantaneous peaks.
Digital is different, because it has a hard upper limit, so is referenced to the peak (and limiting) value. If digital had a "0VU" it would be somewhere well below rated (reference) level, to allow for peaks. For example, a digital recording with 20dB of headroom allowed above "average" (VU meter ballistics weighted) through a conventional CD player rated for 2VRMS output will have the same "0VU" signal playback level (200mV) as a vinyl disc played through electronics of sensitivity 200mV for 5cm/s or tape head electronics of 200mV for 185nW/m (or some number close to that - it varied a little).
So digital players are rated at their peak possible output, rather than some averaged signal value, and only indirectly comparable. Everything before the volume control needs to accommodate these levels; everything after the volume control lives in its own world.
All good fortune,
Chris
Thank you everyone for your opinions. Let me clarify some of the points of confusion:
First, I'm not designing a tape head preamplifier, but rather a stereo preamplifier with several line inputs and two outputs: main output (supposed to be used as input by audio power amplifier) and tape recorder, to drive the input of the tape deck. Please don't confuse with tape header preamplifier, which is driven from tape head directly.
Next, there is incorrect wording in the "monitor" switch description
The third point that I would like to emphasise is ergonomics. I see two issues here:
First, I'm not designing a tape head preamplifier, but rather a stereo preamplifier with several line inputs and two outputs: main output (supposed to be used as input by audio power amplifier) and tape recorder, to drive the input of the tape deck. Please don't confuse with tape header preamplifier, which is driven from tape head directly.
Next, there is incorrect wording in the "monitor" switch description
Instead I wanted to say that monitor switch will be routing either tape recorder output, or tape recorder input (which is in its turn the signal selected by the main channel selector) signal to the main output of the preamplifier. @kevinkr and @techtool have got it right.
The third point that I would like to emphasise is ergonomics. I see two issues here:
- A 10-30 mV (nominal, here and further in the text) tape recorder input driven with 150 mV signal or higher will severly limit the record level controls on the tape deck. For example if their scale is from 1 to 10, they will be only operated between the 1 and say 3, to get input signal at around 0VU. I consider it as ergonomic issue, because it is much more comfortable to operate pots on their full mechanical angle. Easier for the wrist and fingers, lower demand for fine motor skills.
- I would like preamplifier to work well with variety of tape recorders. Quick research shows that their line output do differ in levels, therefore the classic approach used in preamps where tape recorder output is taken as one of the inputs, which is enabled by a separate "monitor" switch without any gain adjustment will suffer from level mismatch and will result in signals reproduced with different volume levels, which is an obstacle for the main purpose of the switch - a mean for signal monitoring.
Any 3-head tape recorder has a MONITOR button (Tape-Source), this switching retains a constant level on a line output (if the tape recorder is correctly adjusted). I see no reason to duplicate this switch in the preamp. I cannot recall such a switch in some preamp. But there is a selector of the rec out. This works similarly to the input selector, but for the output signal for recording on a tape recorder.
As for signal levels, in reality all analog sources have different output levels. For good ergonomics, the volume should not change when switching inputs. Therefore, in my preamp, I added normalization trimmer at each input. For a digital volume controller, it is easy to memorize the normalizing coefficients for each input.
As for signal levels, in reality all analog sources have different output levels. For good ergonomics, the volume should not change when switching inputs. Therefore, in my preamp, I added normalization trimmer at each input. For a digital volume controller, it is easy to memorize the normalizing coefficients for each input.
First paragraph :
This is not true, every full function amp or preamp used to have tape monitor switches, even for two decks frequently.
This only changed some time ago when private recording was not so important any more. The record out selector is a
relatively new phenomenon.
This is not true, every full function amp or preamp used to have tape monitor switches, even for two decks frequently.
This only changed some time ago when private recording was not so important any more. The record out selector is a
relatively new phenomenon.