R&D Info and Contributions Welcome RE SWLP 9.0SE (Vinyl)

modwright · « **on:** 18 Apr 2006, 04:05 pm »

We are finallizing the design of our new phono stage - in both the SWP 9.0SE and the SWLP 9.0SE.

Alan Kimmel has contributed greatly to this design and he and I are now debating the method of cartidge loading and, specifically, whether to allow for variable loading.

The issue is this:

Fixed cart loading means that the load resistor is actually soldered in place in the circuit.

Variable loading means that different values are switchable via relay, dip switch or other mechanical or electro-mechanical contact.

The issue is that the low level signals of MC cart's, potentially as low as .05mV to .1mV output are VERY susceptible to noise pickup. At this signal level, things such as oxidation on contacts, resistor self noise, etc. actually impact the signal. Considering this, the risk of oxidation on any switch contacts at the input to the MC gain stage, threatens the audio signal quality.

High quality resistors are used, to assure the lowest possible self-noise, and the circuit is already designed to account for low noise also, with multiple active parallel input devices, etc. The load setting switch is a possible culprit however.

So, the question is, would the customer rather have us set the load from the factory - to order? Does it make sense to use a 'Universal' load setting for MC carts, such as 100ohm or 1K? The MM setting will be 47K and there will be high (MC) and low (MM) gain settings. A high output MC cart could be used with the MM setting and may prefer 47K load. A low output MC cart however will likely prefer a lower load setting.

Please share with me your feelings about this. Specifically, I am wanting to know how important variable loading is to our customers. I also want to know what is a 'typical' value for MC cart loading. Is the risk of signal degredation worth the convenience of user cart loading?

Thank you, I look forward to your input. I am trying to design a world class product here that will not be astronomically priced.

Dan W.

Gordy · « **Reply #1 on:** 18 Apr 2006, 04:34 pm »

Why not a user selectable plug-in 'module'? The pins could be easily cleaned periodically or treated with SST/Silclear... I've connections and tube pins that are oxide free 2 years after being treated!

JerryH · « **Reply #2 on:** 18 Apr 2006, 07:37 pm »

I like Gordy's idea, but if not that, then fixed. Make the resistors as accessible as possible so that we can change them, or have someone locally do the solder work. That way, you eliminate having to send back to you just to change the load.
Switchable loads would be nice, but not at the expense of quality, IMO

woodsyi · « **Reply #3 on:** 18 Apr 2006, 08:54 pm »

Right now I have 2 carts that need 100 ohm and 500 ohm. I use two different phono stages with fixed load at those values. So those at the moment would bet the values I would need. I think Gordy's idea is good if it mechanical connection is feasible with tiny resistors. Otherwise I think I would like a few discrete selectable values including 100 and 500.

I would also like 2 inputs so I can down size to one phono stage but still keep two TTs.

AB · « **Reply #4 on:** 18 Apr 2006, 08:54 pm »

How about jumpers on the PCB. No need to solder leads, just the pads.
The PCB would have a range of values in place.

The factory chooses a value based on converstions with the buyer and solders the jumper. If the buyer or a subsequent buyer needs to make a change they simply solder wick the old jumper and solder the new jumper.

modwright · « **Reply #5 on:** 19 Apr 2006, 05:17 pm »

I have been speaking with other designers about this too and there are ways to lessen the effects of contacts - or rather, there are better devices to use.

The phono will have two inputs at least, for multiple arms, etc.

I like the idea of solder jumpers on the board, but they would still be wire jumpers as I don't like the idea of even a small solder bridge being the elecrical connection without solid wire.

I really appreciate the feedback and will share more as the design progresses to completion.

Thanks,

Dan W.

Bill Baker · « **Reply #6 on:** 19 Apr 2006, 05:33 pm »

Hi Dan,
How about using a high quality selector (no toggles) internally to allow for different loading. Keeping the selector internal would allow for the shortest possible leads while also maintaining some versatility.
I think if you have only fixed values, you will be spending too much time changing these as people experiment with different carts.
I for one don't mind a fixed loading but not everyone will be capable of changing these resistors on their own.

Just my 2 pennies.

modwright · « **Reply #7 on:** 20 Apr 2006, 05:01 pm »

First of all, I didn't address the possibility of the loading plugs. This is a good idea and is done by Whest.

RE the use of a high-quality selector switch, this is also a good possibility. Greyhill selector switches are very high-quality and I believe have sealed contacts. This could either be done with a knob on the back or from the front with an extension shaft and coupler. If the latter was done, care would just have to be taken that the extension rod, etc., not pick up RF.

I am leaning towards having variable loading just because I believe it will make the unit more flexible, but I am going to listen to units configured both ways to verify if there are any audible compromises.

Thanks,

Dan