Level explanation

[mhennessy]

I'm sort-of familiar with this stuff

As has been pointed out, 0.775V RMS is 0dBu. Yes, it's the same voltage that 0dBm would be when working into 600 ohms - that's where it came from when we transitioned away from the 600 ohm world. The "u" stands for "unspecified", meaning the impedance is not defined - it is a voltage measurement, not a power measurement. dBu is the universal standard in professional audio.

Yes, there is a 4dB offset with a VU meter. 0dBu gives a reading of -4 on the VU scale. Hence, to get to 0 on the VU scale, you need +4dBu, as stated in post #17. That's because VU meters stop at +3, but programme peak levels are usually around 8dB higher than line-up levels (different standards exist).

dBr is something else entirely. Best forgotten about for this conversation. Just like the other standards that have come up so far - not to mention those that haven't yet. It's complicated enough!

The VU meter is a relatively simple beast that indicates average signal levels. Not all that useful in a world where we care about peak levels saturating tapes and over-modulating transmitters, so that's why the PPM meter is usually used in preference. In typical professional circles, the VU meters might used to establish the 0dBu reference levels, but PPMs are used to get the levels correct with programme material. Because of the non-linear scale of a VU meter - expanded at the higher levels - it's actually quite good for setting reference levels with steady-state tones. But that's all. At least this machine supplements the VU meter with a peak LED.

What matters here is the relationship between input and output levels, the readings on the VU meters, and the flux density on the tape. As the machine has been professionally set up, then I'd suggest that you solve your problem using one of these:

http://www.canford.co.uk/CANFORD-PRO...-Free-standing

This sorts out the balanced to unbalanced connections, and there are multiturn presets to tweak the levels. Yes, the price is a bit eye-watering, but you're playing with the big boys now Alternatively, look out for a second-hand example, or build one yourself.

EDIT: I started this post much earlier this evening, and have just seen the latest post. From what you've said, I can't decide if you've undone the good work that you've paid for, or if it wasn't done correctly in the first place. Either way, it's not uncommon to find that the output of the phono preamp is low-level (the specifications in the manual are incomplete). In the domestic world, there are no agreed standards for audio level interchange between sources and preamps and preamps and power amps, beyond a vague "CD players should be 2V RMS at 0dBFS". It's complicated because you need "gain in hand" to cope with low-level sources and quiet recordings, yet you mustn't overload any circuitry before the volume control when fed with CD players and the like.

In short, of all aspects of audio, you'd think that level would be one of the simplest questions to "put to bed". In practice, nothing could be farther from the truth.

[mitajohn]

dBr is something else entirely

Sorry for the typo...I meant dBu.

[Radio Wrangler]

As the old saying has it:

"The nice thing about standards is that there are so many to choose from."

David

[Herald1360]

And I've learned about dBu before sounding off about microwatts (if dBm is dB wrt 1mW, why isn't dBu dB wrt 1uW or even 1uV?) I guess that would have to be strictly dBμW or dBμV.



Now, where are the dog biscuits?

[Radio Wrangler]

dB with respect to 1 microvolt are dBµV but often the Greek lower case 'mu' is replaced by the lower case Roman 'u' for convenience on computers and necessity on typewriters.

Because the decibel is defined as a ratio of two powers, whenever it is used for voltage ratios between different places, you have to worry about impedance levels.

Wire up an opamp for a voltage gain of 20 and we all say the gain is 26dB, but the input impedance is huge, the output impedance is tiny, and the available power gain is immense.

dB often get used with unstated assumptions. A lot of times the right assumptions are made by the recipient of the statement. Sometimes it comes out in the wash, but when noise floors and power capabilities are involved, you have to be absolute in terms of power.

dBµV into 50, 75, 600 Ohms?

David

[Linnovice]

I don't think I've undone any good work paid for and I'm not arrogant enough to say it wasn't done correctly. I think it was set to the 'professional' level and when I queried it I was given the 'stock' answer. At the end of the day I now have a machine that records and plays to a sound quality I am very happy with. I tried recording some classical guitar from vinyl last night. The resulting recording was extremely good. That, I could not do before without an inordinate amount of background hiss.

[mhennessy]

Plain text can be hard to interpret, but it appears that my previous comments might have been taken as criticism - which was not my intention. Then again, perhaps I'm falsely seeing something that isn't present in this message. Either way, I'm sorry if I have offended.

The fact is, the machine was professionally aligned to produce the correct magnetic flux from the input signal, and to produce the correct output voltage from a certain amount of magnetic flux. Obviously, the gain of the record and replay amplifiers were the key adjustment here.

Now, you're using a domestic amplifier that appears to work at around 200mV. As I said, it's hard to be sure from the typically vague and incomplete specifications (quite typical for domestic gear).

As a result, it appears that you have adjusted the gain of the record amplifier to cope with the low level coming from your phono preamp. OK, I'm not absolutely sure if this is the case, but logically, it's the only thing I can think of that would solve the problem.

If I'm right so far, then yes, you have - to an extent - undone some of the work. I wasn't implying that was a bad thing, but yes, I could have chosen my words a bit more carefully. Really, I was seeking clarification about what you'd actually done.

For background, allow me to explain where I come from:

Imagine a broadcast centre with many tape machines installed in many different areas, and imagine for a moment that one of these areas is your setup...

We have two ways to account for the level discrepancy:

1. We could use a "pro-pack" and adjust the gains
2. We could tweak the gain of the record/replay amps in the machine

Now, if we choose option 2, we have a problem. What happens when the machine gets moved to a different area? In a broadcast centre, you'd have a spare or two, and machines get swapped around all the time as faults occur and routine maintenance happens. If every machine had to be differently adjusted for each operational area, it would be a total nightmare. Been there, got the t-shirt . That's why the professional approach would be option 1 - to ensure the installation itself accounted for level differences, and the tape machines would then be standard "black boxes".

Anyway, I hope that explains why my instinct would be to correct the problem externally with a pro-pack, meaning the machine can be left in standard condition. Other solutions are possible, and perfectly pragmatic.

All the best,

Mark

[Radio Wrangler]

A tape machine is useable if the record amplifiers are set so that the flux they produce on the tape is at the chosen standard level when the meters give the correct indication for that level. This allows the gain (of stages before the metering take-off point) to be adjusted to compensate for quiet preamps of quiet source material or quiet performances (or noisy)

The issue you can sacrifice arises of what is a suitable level for fluctuating stuff like music or speech, but this issue is still there even if the machine had been calibrated all the way from input volts through to nanoWebers/metre on the tape.

No recorder can capture everything, so the art lies in deciding what you can afford to sacrifice, and what that sacrifice buys you in other areas.

Of course, if you're using the recorder to tape from vinyl records, CDs or off-air, someone has already had to limit the top end of the dynamic range to suit that medium, and a good tape machine ought to be able to manage all that is conveyed.

One real use for a tape recording chain that has been calibrated is to match its level up with the peak reading metering of a good desk. The crude non-peak metering in the recorder is used with a test tone from the desk to set up this accordance.

David

[mhennessy]

Yup - I'd agree with that - indeed, I've already made some of those points earlier

It's worth adding that scratches can rob headroom from a vinyl disc, and given that tape and vinyl have similar dynamic ranges, this is a problem. Much as I love tape, it's not an archival medium!

I remember being forced to get my head around this really quite early on. When I was about 14, I was attempting to build a tape recorder. I had a mechanism mounted in a nice case - in essence, I ripped out the electronics from a machine and thought I'd have a go at doing my own. I didn't expect to improve on what was already there - obviously - it was purely a learning exercise.

Pretty quickly the replay amps were built. It sounded great - if I say so myself. But how did I know if the EQ was right? I remember playing about to give a bit of HF lift, which made my old tapes sparkle, but was it accurate? All my pre-recorded tapes sounded quite different to each other, so what was right? A friend's dad had an expensive cassette recorder at home, so I got him to make a "reference" recording, but that machine had a different azimuth setting to the one that I'd made all my recordings on, and lining up my machine to match that one also meant my pre-recorded cassettes sounded worse.

At this point, I realised that I needed genuine line-up tapes. Such things were beyond my pocket, so I had no option other than to give up. It remained a playback-only machine for use in my workshop.

Before deciding to give up, I realised that I'd have exactly the same "chicken and egg" situation with the record amplifiers. Until the playback was perfect, how could I begin to produce record amps? And if my replay circuit had an artificial HF lift, my tapes would sound dull on another machine. It's a total minefield, and given my age and non-existent budget (and no test gear), I was on to a loser. But even so, it was a really valuable learning experience that set me up nicely for later life...

A couple of years later I solved the problem by getting a Saturday job and buying a Technics cassette deck in a sale (£60 - bargain). I still have it. This thing makes astonishing good recordings, and is still good today, 25+ years on. Having had that little battle previously, I could really appreciate the work they did in Japan to get what they did from cassette tape.

Cheers,

Mark

[Diabolical Artificer]

I struggle with dB* specs still. It doesn't help that some "pro" mixing desks specs are given as -10dBV for instance, whilst most "pro" gear is true "pro" with input specs given as +4dBu.

When using this with a consumer bit of kit like an amp, where the specs are sketchy, it makes connecting them all up without signal loss or overloading inputs a challenge. You,ve done well, Mike to solve your problem.

Andy.

[Linnovice]

Thanks for those kind words Andy and Mhennessy, no offence was taken or given. I've learnt a lot from this thread so I owe and thank all the knowledgeable contributors for taking the time and effort to pass on their hard learnt knowledge. Even though most of it went over my head .
Mike