78rpm restoration - DC7 steps

[stefano]

Hi, I use DC7 for 78rpm restoration. I usually transfer the R&L audio channels with a RIAA Yamaha preamp. I still have some doubt: when is it better to trasform stereo to mono ? When is it better to apply equalization, e.g RIAA reverse with a specific turnover ?
In the past I was used to apply immediately the R+L transformation (Mono), then I applied the reverse RIAA equalization, but I know that someone suggest to use a different step sequence (e.g. use the impulse noise filter first). What's the better way to do the job ? Thanks a lot !

[DougMac]

My workflow

Stefano,

Here's what I would suggest:

Record in stereo

Apply Reverse RIAA and appropriate turnover using VPA.

Declick and decrackle using whatever method you like.

Check for any residual clicks that may take manual interpolation. Here's where being in stereo helps. Sometimes the click is on just one groove wall. You can use manual interpolation on the left channel (Shift I) or the right channel (Ctrl I) independently. That way when you convert to mono, you mix in good content at the click point.

Use the continuous noise filter. I take a sample of the noise. I then turn on the bypass and use the threshold buttons to move the blue line below the red noise footprint until I hear little or no content. I then grab the squares on the higher frequency end (above 8khz) and drag them above the red line. That removes the hiss above the frequency range of the content.

Then "sweeten" to taste with EQ or VVA or whatever else.

As the last step, convert to mono.

I suggest you invest in a flat phono pre-amp. Reverse RIAA works OK, but I've gotten much better results now that I'm using a flat pre-amp.

Doug

[Audyossey]

...Apply Reverse RIAA and appropriate turnover using VPA.

I've been wondering about reverse RIAA with pre-mid-50s 78s. Seems to me that reversing the curve wouldn't produce a flat response with them. After all the records themselves weren't recorded with RIAA. Reversing the curve eliminates the effect of the preamp, but wouldn't there still be anomalies where the RIAA curve and the original equalization are mismatched? Furthermore, you couldn't remove those with a reverse Columbia curve or whatever because what you have left is neither fish nor fowl. Altogether, another argument for flat preamps.

HB

[Craig Maier]

If you transferred any particular record (be it pre mid 50s 78s or other wise) and you used an RIAA preamplifier as the front-end, the reverse RIAA process will leave behind only the recording curve encoded on the record. So, if our theoretical mid 50's 78 had a European Turnover of 250 Hz, that would be what is left behind. All that needs to be done at that point is to apply the correct European turnover to the record.

So, if you have DC7, the Virutal Phono Preamp has the common conversions built right in there, so it is a snap to do. Even if you transfer an early Columbia LP through an RIAA preamp, you can select the correct conversion in the VPA to produce the mathematically correct result.

Note: Of course, if the mastering engineer did not use a prescribed encoding EQ curve, it will be impossible to accurately reverse it.

[DougMac]

Before CDs

Back in the glory days of LP's, a considerable amount of effort was put into designing good phono pre-amps. Audio magazines wrote review after review of preamps and receivers and spent a lot of time discussing how the phono pre-amp sounded. People fell into various equipment camps based on the phono pre-amp. I bought NAD equipment mostly based on the phono stage.

Well, there's only one RIAA EQ standard and if all manufacturers strictly adhered to it, the phono stage would theoretically sound identical from one pre-amp to the next, right? Well by accident or intent, they didn't sound all alike.

I'm sure the reverse RIAA filter in DC7's VPA precisely follows the standard. Unfortunately, there's no easy way to compensate for the intentional or unintentional coloration in the pre-amp we happen to be using. That's why I think it's better to start with a flat phono pre-amp.

I have the ability to record with either. I'll have to do some testing with reverse RIAA. I've already compared results of LP restoration using an NAD phono pre-amp and a flat pre-amp using DC7's RIAA EQ. I can hear a difference and prefer the flat pre-amp/DC7 EQ. YMMV

Doug

[Craig Maier]

A very strong reason for using a flat preamplifier that has not been discussed too much is an overall reduction in distortion. Why is the distortion reduced?

Well, there are two basic hardware RIAA preamp architectures; one employs the reactive circuitry in a feedback loop (3 time constants) while the other uses passive buffered time constants wired in a cascaded chain. Both methods will yield more distortion compared to a flat preamplifier.

Why?

Let first look a a feedback RIAA preamp circuit. A portion of the output is fed back to the input via the reactive circuit components (Resistors and Capacitors generally speaking) to mimic the reversal of the RIAA curve. More feedback is applied at the high frequencies while less feedback is applied at the lower frequencies. So, the gain of the system increases inversely with frequency. But, the harmonic and intermodulcation distortion are also related to the amount of negative feedback being used around the amplification stages. Since the negative feeedback is not constant, neither is the distortion (IM and Harmonic). In fact, the distortion of such a circuit will be higher at low frequencies than it is at the higher frequencies because of this characteristic. That translates into a what some people refer to as "muddy bass" (and some other people refer to it as a "fat" or a "round" bass).

In a buffered preamp design wherein each of the three time constants are isolated by a buffered gain stage, the drive signal level applied to the various stages is variable as a function of frequency. Since non-linear distorion is a function of drive level, the distorion will also be a function of frequency (not a constant).

In using a flat phono preamplifier (a good one having a very large gain x bandwidth product), the distortion will be almost constant across the entire audio band. The reason for that is that the return ratio (or feedback) used in the amplifier is almost conatant over the audio spectrum; it not a variable. You can sort of tell that is the case when you look at the specs for the CTP family in that the bandwidth of these circuits is well beyond the audio spectrum. That implies that the only reactive (compensation components) have their time constants way past the audio spectrum, thus the loop gain of these circuits is almost completely constant throughout the audio spectrum. That fact yields constant distortion rather than variable distortion found in hardware implemented RIAA circuits; in the CTP family, the distortion is not only constant but extremely low.

The net result should be a cleaner, lower distortion record transfer having little or no color. Of course, if you want color, then this may not be the way to go for you, or you can add the color back in later via algorithms like the VVA, the Dynamic Noise Filter Exciter, the Punch and Crunch Effect, etc.

I hope that this helps clear things up a bit (no pun intended).



Notes: Here is a bit more on the control loop theory that surrounds this topic. Practical amplification devices contain two fundamental mathematical systems. The first is the forward gain which is often referred to in control loop theory parlance as -A (or phase inverted forward gain). The second term is B (or Beta) which is the systems feedback {return ratio}. It is impossible to construct a gain block (-A) that does not have considerable non-linearities in it transfer curve, especially when one views the large signal model of the system. But, it is very easy to create very large values of -A using modern devices. To compensate for the non-linearities and limited frequency response, a lot of Beta is usually applied to the closed loop system. That is why modern circuits can exhibit very low levels of distortion compared to older circuits - - - the ease with which very high values of forward gain x bandwidth products can be found for practical values of -A.

The closed loop gain of a system is roughly -A/B while the loop gain (and stability criteria) are usually measured in terms of -A x B both in phase and frequency. -A x B must always remain at no greater than unity when the phase shift of the system approaches 180 degrees (Nyquists Criterion for Control Loop Stability). So, one often endeavours to make this product as large as possible in the passband of a system, and rolling the system off at some point at 6 dB / Octave through unity loop gain crossover in order to meet Nyquiests Criterion.

If there are any questions about this, I would be glad to amplify (no pun intended) on these comments.

[Geebster]

Yes, Craig. When I was researching flat pre-amps, I stumbled upon an electronics website that outlined in detail, but not as accurately as you, the differences between the differing preamp designs.

It was just one more piece of the puzzle that was nice to know before I purchased the preamp.

GB

[Craig Maier]

Of course, one of the greatest benefits of using a flat hardware approach is that the EQ is performed via a computer, which is almost mathematically perfect. There will be no variations in the frequency or phase response between the left and right channels due to resistor and capacitor tolerances. Also, the curve is in essence the theoretically perfect curve; it is the standard by which any other implementation of any particular EQ curve should be compared.

I think that it is the only way to fly (unless one enjoys the aberations produced by imperfect processes).

[stefano]

Stefano,

Here's what I would suggest:

Record in stereo

Apply Reverse RIAA and appropriate turnover using VPA.....

Many thanks Doug ! Yes, I know I would be better to buy a flat PA, but I bought that Yamaha C-50 because it has the possibility to compensate the RIAA EQ by selecting turnover and rolloff with 2+2 tone/dB knobs that regulate volume at a specific frequency (when I transfer records on my PC I use "disk direct" function). This overcame the need for a graphic EQ.

In DC7 I do not use VPA but Paragraphic EQ: as far as I know the result should be the same, with more turnover choices, not only 250 and 500 Hz.
I do not know if it is my feelings, but when I first reverse RIAA and apply the requested turnover in a second step, with one of the curve in Paragraphic EQ, the bass frequencies seems to be more "warm" than a single pass reverse, e.g RIAA+250 Hz turnover in one step. Did you never notice it ?

Regards

Stefano

[Craig Maier]

It is worth pointing out that the VPA uses closed form math to create the curves whereas the presets in the Paragraphic EQ do not. Thus, they will sound somewhat different. The mathematically precise EQ curves are those produced by the VPA.

[Jan Myren]

How to "sweeten" to taste with EQ and VVA

You wrote:
"Then "sweeten" to taste with EQ or VVA or whatever else."

What settings to you regularly use for EQ and VVA? Any of the pre-sets??

Best regards
Jan

[Jan Myren]

How to record a 78 rpm into the PC?

HI!
You are all talking about a flat pre-amp. Where do I get one of these? Will the classic "Owl 1" or the KAB Vintage Sound Prosessor be off any help?
Look at
http://www.kabusa.com/frameset.htm?/phonpre.htm
I use one of these, find the correct curves according to the age of the record, and then records it straight into the PC with that certian setting. Should I do the recording in another way, maybe not use this box at all in the recording process for that purpose??
Hearing from you

Best regards
jan

[Craig Maier]

No, - - - you loose the benefit by using an analog implementation of the various curves. A flat preamp has no curves by definition. The software adds the curves in later with mathematical precision.

Goto Tracers website and you will find a few flat preamps:

www.tracertek.com

[Jan Myren]

OK, which one do you reccomend?? I will use it for 78's, LP's and 45's.....

[Craig Maier]

There are two versions - - - one which uses a 12AX7 dual triode as the front - end stage (with op-amps used as post-amplification stages) and the other which is strictly op amp based. So, it depends on your taste.

I prefer the Op Amp based one, for what it is worth. My logic is that I can always go back and add in "tube sound" later using the VVA (Virtual Valve Amplifier) if I so desire. It is also lower in price than the tube version, which is an attractive feature. But, the tube unit looks more aesthetically pleasing, I think.