I was responding to your suggestion of using the 500 series "lunchbox"
EQ modules in your post below and, why it wouldn't be the best
application of those devices.
As far as your question of applying passive EQ after the preamp....sure!
I'm guessing this is what the KAB pre-amp does. Not sure though cause I
haven't seen schematics on that particular product. I'm basing my
assumption on the ability to access the EQ section at line level.
Regarding the preamp I plan to build, the turnover and HF roll-off
sections will be passive but in front of different gain stages within
Corey Bailey Audio Engineering
On 4/19/2015 1:36 PM, Tom Fine wrote:
> Hi Corey:
> Why does the EQ module have to have active EQ? Why not take the
> pre-amplified signal from the preamp module, do passive EQ and then
> end the EQ module with a gain stage to bring it back up to line level?
> -- Tom Fine
> ----- Original Message ----- From: "Corey Bailey"
> <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Sunday, April 19, 2015 2:03 PM
> Subject: Re: [ARSCLIST] "Best of both worlds" disk preamplifier
>> 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
>>> 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
>>> 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
>>> 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
>>>>>> 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
>>>>> of the transient--if the A-D converter can accept it without
>>>>> 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.
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