Diff. in input impedance?

[steel1953]

I've been reading where crystals have a really high impedance. Alot more than a Shure CM or CR?
If so, I've read where one thing you do for a harp amp is put a 5.6M resistor in place of the 1M on the input. Would you use some other value for a different element? Is there a way to measure, and optimize the input for your mike?

Just curious......................Brian

[MWJB]

1 meg will still work with crystal & ceramics but 5-10meg max will be better for just about any Hi-Z mic. Because CR/CM & dynamics have lower DCRs there shouldn't be an advantage to going over 1 meg, but listening tests tell me that there is. It's more a matter of the instrument that you are playing rather than the differing mic elements.

The only exception I have come accross is the Shure 520DX, use 1 meg max with these.

[6267]

The other option of course would be to use a matching transformer. A number of companies such as lundahl, hammond, etc. sell these, and they can be passively mounted in a little project box with the appropriate jacks for in and out. Old tube PAs such as the ones we hear on the great harp recordings often did this. My experience has been that they tend to tame the high-frequency shrillness of the crystal element, as well as somehow give it more bottom end (perhaps because of the better impedance matching). The best option would be a box with two mic transformers, one step-up for low-impedance mics, and one step-down for crystal mics.

[Sock Puppet]

You won't get a transformer to match the input impedances needed by a crystal mic (just did some sums and I think I could be wrong there, sorry).

Best bet is to head on over to Aron's forum and look for the Mosfet booster that Jack Orman popularised.

IME, that booster and its variants will bring out the sweetest sounds regardless of what you plug in, mic or guitar or whatever.

S.

[MWJB]

Several hundred K, or more, is still an acceptable match for a crystal or ceramic element..it will still function normally - it's just that several meg may be preferable.

"Old tube PAs such as the ones we hear on the great harp recordings often did this." Many old tube PAs in the early 50's were grid leak biased, this has the mic seeing a very large load, the #meg resistor at the tube grid is predominantly there to bias the tube. Grid leak can sound fine with crystal & ceramic mics, but I've never been keen on the sound with other types.

Of the old tube PA schematics I've seen (admittedly mostly Mascos), I don't recall seeing any with transformer inputs.

Just one other thing...which "great harp recordings" are you referring to? I'm not sure that there are many old recordings that could be definitely attributed to PAs (maybe some late 50's Cotton with Muddy, Snooky Prior with Johnny Young, possibly a handful of Little Walter cuts?) rather than guitar amps/combos (I might be accused of splitting hairs here as in the early 50's either were usually cathode biased with paraphase, or cathodyne PIs)...there's probably been more harp recordings made with tube PAs/amps inspired by old PAs (like the SJI/SJII) in the last 15-20years, than at any time before. Though, of course, the likelyhood of old tube PAs being used live & on the road in the old days is very high.

[6267]

I was mostly speaking to the old and well regarded little walter recordings, but I assumed that in many cases (especially where musicians are singing and playing harp that they were just cupping the vocal mic which was probably on the PA). Recent stuff is almost always *not* the PA...I have a PA from the early 60s which has two XLR inputs, one is straight into grid leak channel, the other is 200 ohm - 10,000 ohm transformer coupled into the other grid leak channel. It sounded great with the crystal mic on the grid leak channel, but I figured that you could also have flipped the transformer wiring to get an even bigger input impedance if necessary (magnifying the grid leak resistor by 50x). An even lower ratio would probably be preferrred.

[MWJB]

Well, on many Little Walter recordings it's pretty well impossible to determine exactly what he was using, lots of folks have views on the subject, but they're impossible to prove/attribute to this or that track. It is perfectly feasible that he did indeed use old tube PAs to record (there are certainly a few good candidates to my ear, "40 Days/All Aboard", "Just to Be With You/Don't Go No Farther" with Muddy spring to mind, as do "My Babe/Thunderbird" - but I'm merely speculating, as is everyone else), as well as play out live.

However, to attribute a specific style, model or brand of amp to any one recording would seem to be beyond the realms of reason and largely (totally?) conjecture. Bear in mind also that recordings that feature either tape delay, or a reverb chamber (even both), re-EQ the source, affect gain and make it very difficult indeed to make any hard & fast conclusions either way, or indeed to even clearly hear the source as it was picked up by the studio mic.

There are numurous LW recordings that most likely were not tube PAs of the types often linked to LW, simply because these recordings have amp tremolo on the harp. Tremolo if ever fitted to cheap PA heads (I've yet to see it, but indeed I certainly haven't seen anything like ALL old tube PAs, far from it) was a relative luxury & PA heads (typically cathode biased, low parts count, low voltage and underfiltered) were a cheap, rather more utilitarian alternative to combos.

Walter and his contemporaries would undoubtedly also have sung through their combo amps (as did John Lee Williamson fron the mid 40's onward according to Muddy), in fact this was pretty common practice right up until the mid 60's. It's important to differentiate between what we commonly perceive to be a typical PA today (SS several hundred W minimum), compared to the low powered, 20-35W tube PAs of the 50's...they are very different animals (as I'm sure you know).

Rick Estrin has recorded with a Masco PA in recent years, Kim Wilson recorded with a Sonny Jr I amp that was inspired by a Masco PA, that's just off the top of my head - plus guys like Skip Simmons have had a healthy trade in refurbing old tube PAs for harp & guitar players in recent years.

[6267]

Not to get offtopic too much , but my main point is that high-Z microphones work a lot better when using the impedance matching transformers they were often designed to use. In reference to Little Walter, I agree–I've read quite a few conflicting accounts of what he was playing through, and what kind of harp in which songs etc. (I've heard he sometimes played a chromatic with the slide in for entire songs!) While I certainly agree, and there has been quite a bit of confusion about what little walter did for his sound, I am basing my assumptions of the following:

Most performance halls etc. would have a large, powerful sound system, perhaps even running 807s or radio transmit tubes for the main speakers. These were not 'PA Heads' so much as house music systems, as were used in theatres at the time (you should see some of the monster theatre amps that were in use from the 30s on). They certainly didn't have tremolo. I have a feeling many artists did not own their own gear, or couldn't always take it with them. I would expect that the house's audio often was used, or whatever could be taken with. So certainly LW didn't play through one thing all the time, nor did many of the bands. Commercial amps at the time were ok for small venues or recording I'm sure, but I still suspect the house PA was used quite frequently as well for vocals and/or harp. I do agree that recording methods often changed the sound quite a bit, making much of the discussion moot. I also know, however, that many of the sure mics from the time were sold to be used with mic impedance matching transformers in telephone repeater/radio line/PA use, often matching the high impedance elements down to around 600 ohms (much of the audio technology at the time was based on phone company equipment, which used a line Z of around 600).
I'll try to find a link with the pictures of a typical theatre PA, I know I saw some pics recently...

[TheTinMan]

1 meg will still work with crystal & ceramics but 5-10meg max will be better for just about any Hi-Z mic. Because CR/CM & dynamics have lower DCRs there shouldn't be an advantage to going over 1 meg, but listening tests tell me that there is. It's more a matter of the instrument that you are playing rather than the differing mic elements.

The only exception I have come accross is the Shure 520DX, use 1 meg max with these.

The Shure 520DX specs say minimum load is 100K. Is that mike specifically designed to go straight into a guitar amp with the typical 1 meg load resistor? What happens when the load is too big?

Chip

[MWJB]

Yes, the 520DX is designedto go straight into a typical guitar amp input, loads over 1meg tend to cause earlier feedback.

[TheTinMan]

Yes, the 520DX is designed to go straight into a typical guitar amp input, loads over 1meg tend to cause earlier feedback.

Thanks!

I may work on setting up the #2 input so that it presents at least a 100K load.

Chip

[bluesman60]

match them

its been my experience that one should try different mic and amp combinations as they all will give a different sound and as far as impedance matching...if you use a high imp. mic you shouldn't need to do anything but plug in and blow,i have about 25 different harp mics and half a dozen amps and every combination is different...also the 520dx is really a bad mic,unplayable in my opinion,find an old one...