Passive IRAA circuit

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marchingband
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Passive IRAA circuit

Post: # 46026Unread post marchingband
Wed Feb 08, 2017 7:49 am

I've been unable to track down a schematic for a passive IRIAA circuit.
I see that FLO had posted one once, but I couldn't find a working link, or a post with it shown.
If anyone could link me to a copy, I would be grateful!
thanks!
yo

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markrob
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Re: Passive IRAA circuit

Post: # 46028Unread post markrob
Wed Feb 08, 2017 8:29 am

Hi,

Here it is:

[img]passive_riaa_20db.jpg[/img]
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powerstrip
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Re: Passive IRAA circuit

Post: # 46034Unread post powerstrip
Wed Feb 08, 2017 3:40 pm

mark -

so wait will this encode the iriaa or riaa into the cut?

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markrob
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Re: Passive IRAA circuit

Post: # 46035Unread post markrob
Wed Feb 08, 2017 4:55 pm

Hi,

This circuit is used in the cutting chain. It boosts high frequencies relative to 1Khz. Since its passive, it has a loss of -20db at 0 hz, so you have to have some makeup gain somewhere in your recording chain. You also have to make sure you drive with a low source (< 1K ohms) impedance and follow with a high input impedance (> 50K ohms) or the curve will be altered.

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powerstrip
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Re: Passive IRAA circuit

Post: # 46036Unread post powerstrip
Wed Feb 08, 2017 5:55 pm

good look as usual mark, I have been looking to install a few of these, though the plugin you provided years back is good, I do it differently now

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marchingband
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Re: Passive IRAA circuit

Post: # 46038Unread post marchingband
Wed Feb 08, 2017 7:32 pm

@markrob Thank You!!!!!!!! SO simple!

I am using a Presto 1C cutterhead, some people mentioned that this Cutterhead does not require a low frequency cut, but there seemed to be some disagreement. What is the prevailing wisdom now on this?
Is this unique to Presto cutterheads?
Does this mean that FLO's schematic would need altering to work well with this Presto?
yo

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markrob
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Re: Passive IRAA circuit

Post: # 46039Unread post markrob
Wed Feb 08, 2017 8:55 pm

Hi,

The 1C and D both have a built in cut 250hz for the 1C and 500 hz for the 1D when used with the recommended resistor in series with the head. If you had the head re-wound this value may be different. If you use the circuit given, you would have to counteract the response in the 50 hz to 500 hz range. Or, design a network that only had the 75 us (2122 hz breakpoint). Here is a quick one I drew. I think this is correct. Note that you would need to make sure you head has the rolloff at 500 hz for this to work well.
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marchingband
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Re: Passive IRAA circuit

Post: # 46042Unread post marchingband
Thu Feb 09, 2017 5:10 am

@markrob thank you for the schematic!
My head has '8 ohms' stamped onto it, which seems odd. I got it from a basement, where it had been for a long time, so if it had been re-wound, it would have been done a long time ago. I don't believe they came stock wound at 8 ohm did they?

Where could I find info about the series resistor you mentioned? I am not currently using any resistor.

thank you!
yo

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marchingband
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Re: Passive IRAA circuit

Post: # 46043Unread post marchingband
Thu Feb 09, 2017 6:27 am

I did some more research, and I see that 8ohms IS one of the stock ratings.
I havn't been able to find the 1D manual that would tell me the resistor value, unfortunately.

Would you be willing to send me your RIAA plugin? I am realizing that, with a series resistor, is a better way to go. If I can figure out what value that resistor needs to be ....
yo

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Gus
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Re: Passive IRAA circuit

Post: # 46044Unread post Gus
Thu Feb 09, 2017 6:38 am

marchingband wrote:I did some more research, and I see that 8ohms IS one of the stock ratings.
I havn't been able to find the 1D manual that would tell me the resistor value, unfortunately.

Would you be willing to send me your RIAA plugin? I am realizing that, with a series resistor, is a better way to go. If I can figure out what value that resistor needs to be ....
Here is
presto.jpg
The manual mentions only for 15Ω and 500Ω Impendance
May i ask, is that normal to rewound a cutterhead out of specifications?
i mean at 8Ω to work for today's amps?
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markrob
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Re: Passive IRAA circuit

Post: # 46046Unread post markrob
Thu Feb 09, 2017 10:17 am

Hi,

If its marked 8 ohms, that indicates its a 1C. AFIK, The 1D heads never came stock with that winding. I've never seen a 1C manual (not even sure if they specified a series resistor for the 1C), but back when I had both heads, I can tell you the only difference was the damping material and the winding. The 1C was in production before the RIAA was in place and I believe 250 hz was a popular low frequency turnover point at that time. By the time the 1D was in production 500 hz was more standard. I Seem to recall needing to put about 2 ohms in series with the 1C wound fro 8 ohms to get the 500 Hz turnover point to work out. But you should experiment. Probably the best way is to plot the response with no RIAA in place on cutting or playback (you would need to build a flat phono preamp) and see where the turnover falls. Start with no series resistance and add as need to shift it to higher frequencies.

If you head has never been refurbished, I would send it out to Todd to get cleaned up. Well worth the cost.

If you want my plugin, send me a PM with your email and I'll send it along.

Mark

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markrob
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Re: Passive IRAA circuit

Post: # 46054Unread post markrob
Fri Feb 10, 2017 2:01 pm

Hi,

I did some playing with LTSpice to see if I could improve on the passive Presto version of the passive IRIAA. Missing in the first schematic I provided, was the boost needed below 50 hz. After some playing around , I came up with the circuit detailed in the attachment below (click to enlarge). Note that the output is at the junction if R1, R2, and R3. It's not dead on, but should be close enough for this application. You may still need to season taste with some EQ.

Mark
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grooveguy
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Re: Passive IRAA circuit

Post: # 46055Unread post grooveguy
Fri Feb 10, 2017 3:21 pm

Hey, guys,

Just happened to see this thread and wanted to chime in. I started out with a Presto 1D, and never quite understood the company's recommendation for a series resistor. I suppose I keep harking back to loudspeaker and headphone practices, where an essentially-zero-impedance, constant-voltage drive to the transducer makes for best transient response, greatest suppression of resonance peaks and, for recording, greatest control over the stylus tip by the amplifier.

There is definitely "mechanical EQ" going on in the cutterhead itself. References in books on the subject refer to "...constant amplitude below [some frequency], and constant velocity above." I think the difference between the Presto 1C and 1D illustrates this, and probably has to do with the mass of the armature.

What I endeavored to do was pretty similar to tape recorder EQ practices, which was my area of expertise back in analog recording days. In other words, you have a frequency characteristic that is defined by a playback curve, and then use whatever it takes to make the overall record/play response flat.

I had three 'calibration' test records, one from NAB and two from CBS labs. Although my playback chain was pretty good (Stanton 681/triple-E), measured response from these records was an unimpressive ±2dB of flat out to 15kHz, with some wild tonearm resonance down at the very bottom. But the three records were very close to one another, so I assumed that all I needed to do was make my lacquers play back just like the calibration discs. So at the specific frequencies noted on the test records I made note of my playback error to come up with fudge factors for overall flat response.

What I ended up with for a record curve was definitely not an "inverse RIAA." It was more like a 3dB/octave rising characteristic from 50Hz to 15kHz. I obtained the record EQ with several sections of active and passive EQ, and still have all the plots and schematics if anyone is interested. Bottom line: the lacquers I cut were within about 1.5dB of the playback of the test records, and sounded good to my ears. I could actually hear 15kHz back then, and attribute 15k response to using a short-shank stylus, which bends less than the Presto-specified long-shank ones. Maybe that head needed the extra leverage of the longer stylus to modulate wide-groove 78s to "jukebox volume." But for microgroove 33s and 45s, the short-shank definitely improved the top-end.

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markrob
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Re: Passive IRAA circuit

Post: # 46061Unread post markrob
Fri Feb 10, 2017 8:32 pm

Hi,

Those are all good points. At the end of the day, you want the playback to be flat. Whatever it takes.

My understanding of the need for the series resistor is to adjust the L/R time constant of the drive coil to the 500 hz point. You have a default L/R in effect due to the DC resistance of the drive coil. If effect, you are getting one part of the recording characteristic for free. As you add series resistance, you push the break point to higher frequencies. The typical time constant is longer in a moving iron design since you can pack much more wire (hence larger inductance) into the drive coils as compared to a moving coil design. The moving iron head can work with much higher stiffness (lower compliance) since the motor can generate more force. This allows you to push the system resonance up into the 8Khz and up range. So you end up with a constant amplitude characteristic until the L/R break point and the head becomes constant velocity up to the system resonance. Mechanical damping pushes the response out past resonance and then falls off.

The moving coil design has a very short L/R time constant and does not come into play until much higher frequencies. However, the stiffness is set lower and the system resonance is set in the 1-2Khz range. In this case, you have constant amplitude response up to system resonance, followed by a falling off of -6db/oct on the other side of resonance. This design needs quite a bit of boost to flatten the response at high frequencies (not even including RIAA).

Mark

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grooveguy
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Re: Passive IRAA circuit

Post: # 46062Unread post grooveguy
Fri Feb 10, 2017 8:52 pm

Right you are, Mark!

What you described is exactly what the series resistor would do. My concern, however, remains with the implication of series resistance ruining the damping factor. Having said that, if the resonance of the head is indeed as high as you indicate, and I would guess that, with the masses involved and the stiffness of those springs, it would be, then by the time you got to 5-8kHz, the inductive reactance of the coils would make the few ohms in series with the head a very small part of the total impedance.

With loudspeakers, of course, you have a comparatively low primary resonance, and series resistance does upset the damping afforded by a low-Z amplifier output. One of the L.A. market Chief Engineers had a situation where DJs were complaining that their new headphones sounded 'tubby.' These were good Sennheiser cans, sounding superb when driven with a good headphone amp. But the headphone output of the console happened to be the +4dBm nominal line output, with a couple of series resistors and a wirewound pad to drop the level to the headphone jack. The effect was that the headphone driver resonance, about 300Hz, soared up to a rather sharp +9dB peak when driven with that output. An outboard headphone amp solved that problem.

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