Hi Tom,

Let's start with the fact that active equalizers make better filters 
than they do gain amplifiers. I proved this on the bench years ago. In 
fact, it caused me to re-think the way I approached recording: listening 
for what I wanted to remove instead of boosting what I thought was lacking.

When boosting frequencies with an active equalizer (API, MCI, Neve, 
Radial, etc.) you get unwanted phase distortion, not to mention running 
out of headroom if you are talking about the amounts required for record 
EQ. The reason is that, from a design standpoint, the vast majority of 
equalizers used in pro audio equipment (at least, all of those you 
mentioned) have the RC components in the feedback loop. Thus, the phase 
distortion when boosting beyond 3dB or so.
In the case of record EQ, we are talking about only boosting the low 
frequencies for playback but, that is the area where phase distortion 
can be most difficult to hear. However, it is certainly measurable.

My $0.02

Corey Bailey Audio Engineering

On 4/19/2015 7:02 AM, Tom Fine wrote:
> I had a thought about a different way to do a phono preamp and EQ 
> system -- as a series of API 500 "Lunchbox" modules. You save some 
> design time and expence because there's pre-made and standardized 
> power supply and connection interface. There might be a deal to be 
> made with Radial or API itself or another 500 series frame-maker to 
> fabricate a custom frame allowing for unbalanced RCA connectors on the 
> back, or you could just include adapters with the modules. The 500 
> series if full of preamplifiers and equalizers, both tubed and 
> solid-state (and hybrids), so this is all within known and done 
> territories.
> I can envision this sort of system:
> 1. a separate phono preamp module. Perhaps such things as tube and 
> solid-state options can be offered. You'd need to build in unbalanced 
> inputs and also direct outs for flat transfers. The tube version would 
> likely be 2-wide (that seems to be what's required for true-to-spec 
> tube voltages plus enough real estate to allow for tube heat-venting).
> 2. a separate EQ module, two channel or perhaps one module for each 
> channel. There would need to be channel-to-channel level and 
> phase-relation controls to allow for a wide variety of vintage disks. 
> Perhaps also some sort of low-frequency phase control, which can 
> sometimes very much help the sound quality while not lopping off the 
> bottom end via a "dumb" rumble filter. A choice of EQ curves, of 
> course. I advocate separate turnover and rolloff controls. Perhaps 
> there could also be a separate super-high-grade RIAA EQ module, maybe 
> with a choice of tube or solid-state topology.
> 3. there could also be an output/routing module. This would be helpful 
> because it could put the power demand for the output line/buffer amp 
> in a separate module (perhaps necessary in 500 series standards), plus 
> the extra real estate could be used for such things as a processor 
> loop (either external or looped to the next slot or slots in the 500 
> frame) and perhaps a transformer output option.
> 4. in a Radial 500 frame, there would still be room for two channels 
> of dynamics processing or mastering EQ modules.
> Being a modular systen, the user could have at least these options: 
> tubed or solid-state preamp; many-curve or super-hifi RIAA EQ module.
> -- Tom Fine
> ----- Original Message ----- From: "Paul Stamler" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Saturday, April 18, 2015 10:27 PM
> Subject: Re: [ARSCLIST] "Best of both worlds" disk preamplifier
>> On 4/18/2015 1:25 PM, Richard L. Hess wrote:
>>> On 2015-04-18 3:07 PM, Paul Stamler wrote:
>>>> Well-designed tube circuits can have transient response as good as
>>>> transistor circuits, and incredibly low distortion as well (with the
>>>> application of negative feedback). I'm sorry, but the idea that
>>>> "euphonic distortion" is an inherent characteristic of tube 
>>>> circuits is
>>>> one that's been carefully nurtured by the purveyors of tube hi-fi 
>>>> gear.
>>>> I'll go on record as saying categorically that it ain't necessarily 
>>>> so.
>>> They key to this is circuit design/topology and paying attention to 
>>> gain
>>> staging through the device as well as component selection.
>>> Tubes can handle rather large voltage swings and if the R/C rolloffs 
>>> are
>>> properly controlled they may actually provide a cleaner representation
>>> of the transient--if the A-D converter can accept it without excessive
>>> noise. Again, gain-staging is of paramount concern.
>> It is, and it's important to pay close attention to load impedances. 
>> Most common tubes (high-mu) have trouble driving lower impedances 
>> like the common 10k without a transformer. Low-mu tubes like the 6SN7 
>> can do it, but they're bigger, use more current, etc.
>> Peace,
>> Paul
>> ---
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