OK, here's a curly one for you electronics gurus !!!!

Just for fun I recently put together a Boozhounds Pre Preamp kit for use with a LOMC cartridge.

This is similar to the well known Le Pacific circuit using 2sk170 jfets and PIO output caps.

When I connected this to my Project Phonobox SE II ( MM mode and 47k ) it would barely make a sound.

I double and triple checked my assembly of the kit but could find nothing wrong.

Finally I connected it to my Graham Slee Gram Amp II and it worked perfectly !!!!!!!

It is a complete mystery to me but there must be something in the ProJect circuit that is incompatible with the pre preamp.

The LOMC cartridge works fine into the ProJect straight into the MC mode or with my vintage Ortofon T-10 SUT into MM mode.

Has anyone experienced something like this or have any ideas why this should be ??

Confused  

Hmm. The only thing I can think of is that the mc loading on the Project is on, even though MM gain is selected. I'll look at the Project settings to see if that's possible.

AMR-iFi R&D

Good to see some electronics DIY there Graham!

These are the problems that will end up as indelibly good learning

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Jun 28, 2017 13:28:32 GMT 12 colinf said:

Hmm. The only thing I can think of is that the mc loading on the Project is on, even though MM gain is selected. I'll look at the Project settings to see if that's possible.

Hi Colin

I was powering the pre-preamp with a 12v SLA battery and feeding the output RCAs into the ProJect using MM mode with no jumpers installed which provides 47k ohms and 120pF. This is the default MM mode.

When I use this same setting on the ProJect with MM cartridges it works perfectly.

Really strange.

Graham - just to be clear....

- the weak signal result is same on both channels, I take it?
- were you careful when soldering in the JFETs? (eg. soldering them in last)?

Check that the SLA batt is supplying 12+V at the PCB (ie when connected to the Boozehound).
Check that the Boozehound-to-Project plug-socket connection is good.
Maybe select 220pF (ie exactly same as the G Slee) with a jumper.

A photo of your Boozehound build?

DarkLantern blog - darklanternforowen.wordpress.com

The only thing I can assume then is that there was an error that's been overlooked and perhaps not obvious. Eg I know it sounds simple but was the power on the MC stage turned back on when you connected the Pro-ject? Interconnects functioning ok? If the Pro-ject functions normally with your Sut I suspect you could rule it out as a problem. It's very difficult to fault find without it being here. Perhaps as Owen says, a pic of your MC build would be nice. If it's running from a 12v battery the voltage on the 2sk170 drain connections should be 6v or so.
If you suspect some incompatibility with the Pro-ject circuit you could always send it here and I can measure it to see if it is functioning normally.

AMR-iFi R&D

Oh yes, and I wonder if you have DC on the output of your MC stage. It should be 0v. If it isn't, suspect is the output capacitor. If the Pro-ject is direct-coupled to the cartridge, which is very common and possible, it may set its circuitry out of operating range. Perhaps the Graham Slee has an input cap to isolate DC. But I don't know it's circuit so I really can't say.
Now here's some even more complicated stuff that's also possible, but would be rare. On your MC stage, is there some form of filtering on the power supply rails, and the 12v battery is isolated by a resistor? If not, when you switch it on, it might send a huge 6v transient into the input stage of the Pro-ject and "hang up" its dc servo error amplifier, if it has one.
What is the actual schematic of the circuit you built?

AMR-iFi R&D

Here is my version of the Boozhound Pre-preamp.

As you can see, the circuit is beautifully simple ( like me ), using obsolete jfets and PIO caps.

My only mods are the LED and an on/off switch.

Connected to my Graham Slee or Art preamp it works perfectly, it just doesn't like the ProJect.

When connected to the ProJect in MM mode at full volume on the amp I can just faintly hear music from both speakers.

Very very strange

Ok, try this. Try switching the power to the Boozhound on with the interconnects disconnected from the Pro-Ject. Then plug it in. See if it works then.
There could also be ultrasonic oscillation happening when you plug the Boozhound in as there is no isolation resistor in series with the output cap. Are you using the same interconnects to plug into the Pro-ject as you are on the other two phono preamps?
Otherwise it looks like I'd need to do some testing here.

AMR-iFi R&D

OK Colin, just tried your suggestion but unfortunately no change.
Yes I am using the same interconnects each time.
Very very strange.
I really only built the kit as an interesting project and certainly didn't expect this sort of frustration.
The cartridge is currently playing happily into the ProJect using my little vintage Ortofon T-10 SUT and sounding very nice.
I have sent off a query to ProJect via the local agents to see if they have any idea what is happening.

Graham - I take it that the problem is same on both ch's?
That suggests that either (i) it's some unusual circuit incompatibility, or (ii) a PSU issue.

As said, I would check voltage entering the Boozehound when connected to the Pro-Ject - >12vdc hopefully
Then I would try another PSU, a wall power-pack or a switch mode supply such as laptop 19vdc supply (IIRC Boozehound specify 12-24vdc?).
(Careful with correct polarity.)

DarkLantern blog - darklanternforowen.wordpress.com

Did you measure about 6v from ground to pin 1 (drain) of the 2sk170 in each channel? With the values of resistors shown the jfet you need for 12v battery operation would have an Idss (drain saturation current) of above 6mA. Most of the 2sk170s I have measure about 5mA so perhaps higher voltage on your power supply is needed to get it to work properly. Maybe try Owen's suggestion of using a higher voltage plug pack. You could even run your circuit with 2 12v batteries connected in series to get 24v. In which case the 2sk170's drain voltage on pin 1 would be (depending on its Idss) between 12v and 18v. If you're doing that make sure capacitor C6 is rated for 25volts or above.

AMR-iFi R&D

OK. Thanks for your advice. Worth a try.
With 12 volts input the voltage at pin 1 ( drain ) is 4.5 volts.
With 24 volts input the voltage at pin 1 (drain ) is 15 volts.
Still no bloody music !!!!!
As mentioned, it works perfectly into the Slee Gram Amp II on 12, 18, or 24 volts DC.

Ok great, the voltages are correct. There's probably no reason to suspect that the Boozhound isn't working properly, especially as it works ok into the other 2 phono preamps.
So perhaps there really is an incompatibility issue here. I would need to measure the phono stage input impedance, but to do it accurately I'd need to hook it up to the oscilloscope and signal generator, not just measure it with a multimeter.
It will be interesting to see what Pro-ject themselves say.

AMR-iFi R&D

No response from ProJect yet !!
Could the fact that the Boozhound circuit is just DC while the ProJect uses 18v AC as a PSU be the problem ?
I'm presuming the ProJect has dual supply with 18+/ground/18- onto the preamp circuit board.

This raises the question for a curious soul like me, why are some circuits just DC with the Ground shared with the -, while others have the Ground and - separate ?
I have a little preamp kit that I bought years ago but have never built that has this +/ground/- configuration. On the printed circuit board the ground connections are common but the - is only connected to the op amp. There are however 0.1uF caps between ground and + and ground and -.
Is one configuration considered to be better than the other ?

Gidday Graham - AC or DC PSU should not be an incompatibility, as each cct will end up internally supplied with DC from its PSU.
Also, as colin mentioned, the output of the Boozehound has a 'DC-blocking' cap that, if working, will allow only AC music signal through.

The 2 types of cct that you mention, are sometimes differentiated as 'single ended' vs 'balanced'.
Usually, ccts are the former, where the +/- signal cct shares its -ve side with Ground - this often called the Common Ground.
Balanced ccts (usually used in 'pro' systems, because of long cables & noise problems) also have +/- signal lines, but the Ground is a separate, 3rd line. Thus, the +/- are sometimes referred to as 'floating'.

For domestic hifi, single-ended (not to be confused with single-ended valves) is 99% most common.

(DC when referring to Direct-Coupling, is another cct term, something different again.)

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OK. Going off at a bit of a tangent here.
On the little preamp kit mentioned,( that I haven't built yet and may never get around to doing ), with the v+/ground/v- terminals, if I was to connect battery + and battery - to say 12v DC but leave the common ground terminal not connected would it work ? If so why bother to have the ground terminal ? ?

(I've not a lot of experience in SS but....) IC chips, opamps etc often require + & -ve voltage supplies in order to function.

Ccts usually operate on DC voltage/s which need to be 'referenced' to a 0V point/s in the cct.
This 0V (or 'circuit ground') usually ends up connected to the Mains Earth (sometimes via a small resistor or R/C filter in order to reject mains noise).
Except in Japan, where they like to 'float' their ccts with no Earth connection (like their 'dble-insulated' appliances).

DarkLantern blog - darklanternforowen.wordpress.com

We are getting into circuit design now :-)
Owen, I think Graham is talking about power supplies only here, not balanced audio signals as in some high end gear and almost all recording studios.
Graham, depending on the circuit, you can power it with either + and - power supplies if it's a typical opamp circuit. If it's a circuit like the Boozhound you only need a + power supply. In that case the signal ground is connected to the power supply ground which is called 0v. The power ground can be erroneously labelled as -ve but because it is connected to the signal ground it is now 0v. The Boozhound relies on coupling capacitors to allow the audio signal output to be referenced to 0v with a resistor.
Opamp circuits are usually constructed with + and - supplies (typically +15v and -15v) that are referenced to signal ground. That means you can eliminate one or more coupling capacitors as the signal is direct coupled to the opamp. Usually the input of the opamp is direct coupled as most opamps generate a bit of dc offset, necessitating the use of an output coupling cap.
The signal ground is at 0 volts and it is considered a 'clean' earth. It is usually connected to the power supply ground with a length of wire, or a resistor, that separates the signal current from the power supply current. The power supply ground is usually considered a 'noisy' ground as it can carry the effects of RF interference and rectifier switching noise.
The Pro-ject runs on 18Vac. Inside, that will be rectified into + 25v and -25v. (That's root 2 x 18 = 1.414 x 18v.) After that it will most likely be regulated with a regulator circuit to +15v and -15v for the opamp. Most audio opamps have a voltage limit of 36v between their power pins i.e. +18v and -18v added together. Some, like the venerable NE5534, have a 44v limit.

AMR-iFi R&D

OK Guys.
On the simple LM833 opamp based circuit shown below will it function with the -15 from the IC connected to the common ground ?
I'm presuming the answer is no or the designer would have done this, or maybe the answer is yes but it could be noisy ??
Third and last question, could it be modified to function this way ?

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No. To get it to function from a single supply of +30v (ideally, but +15v will work too, but the headroom is reduced) you need to replace the 100k resistor after the 47uf cap on the input with two resistors; replace the 100k with 220k, and add another 220k resistor to +ve power 30v or whichever voltage under 36v you have chosen. I also wouldn't be using a 47uf cap. It is probably an electrolytic and a large value chosen so that it has low distortion. I would use a 1uf polypropylene (good) or polyester cap. Also, the LM833 can be upgraded to an LME49860 opamp.
In this case of operation the -ve opamp pin (4) now connects to signal ground and power ground, and the -ve power supply is not used.

AMR-iFi R&D

Thanks Colin.
So I could use a 24v DC PSU I have available.
Just to clarify exactly where the additional resistors would be substituted and connected can you please provide a quick sketch ?
Cheers.

Use this circuit for +24v operation.




high res image hosting

AMR-iFi R&D

Thanks for that Colin.
You are a legend !!
I friend of mine is in need of a basic MM preamp so I will build this as per your recommendations.
The kit provides a socket for the IC so that could be swapped at a later stage.
Cheers

No worries! Let me know how you go.

AMR-iFi R&D

A couple of quick possibly silly questions Colin.
Are the 470uF and 4.7uF caps polarised electrolytic and at least 25v?

Is it recommended to change the cartridge load impedance from the indicated 100k to 47k ?
Is this additional circuit providing filtering via these caps as well as supplying a small DC voltage to the signal input ?
Just curious and trying to learn.

Yes, 25v and above for the polarised caps, except the 47uf on the negative feedback network, which can be 16v and above.
No, the cartridge loading is 50k already. The 100k, 220k and 180k are effectively in parallel, equaling 49.7k.
Yes the resistor bias network with filtering by the 4.7uf cap supplies half the power supply voltage to the opamp input so that both it and the output will sit at (in the case of 24v power supply) 12v. The 0.33uf cap on the output will charge to 12v via the 1M resistor and the output audio signal works around 0v at the RCA socket.
You can make the 4.7uf cap filtering the input bias network larger to get better filtering, just that it has to charge up through the 39k resistor so a larger capacitor will take more seconds to charge fully.
I put in a 100 ohm resistor in series with the +24v power, and a 470uf cap to filter noise from the power supply to ground. If your power supply is regulated with low noise, you may not need it.

AMR-iFi R&D