Crossover for Naim

Kiernan

If you redraw the schematics in a more traditional layout form you will find them easier to understand. In each set of four transistors, one ZTX384 and the ZTX214 form a compound pair emitter follower, with a constant current source made from the other two ZTZ384.

The first set of four transistors and associated components is a unity gain input buffer. This is followed by a passive 6dB/8ve filter stage and a 12dB/8ve active filter utilising the second set of four transistors in place of an op-amp.

The crossovers are therefore 3rd order 18dB/8ve and have been designed primarily for use with Naim SBL and Linn Kan speakers but I see no reason why they should not work well with other two-way designs. A search on the Naim site might elicit more information regarding the use of the Naxo crossover.

Geoff
 
Kieran

Naim uses a cross-coupled buffer used extensively in video circuits, an improved version is used by Levinson. The circit can be improved by current sourceing the emitters of the input pair.
Another approach is to use two complementary j-fets which simplifies the circuit down two active devices per buffer.

Jam
 

ALW

Member
2001-06-12 10:15 am
UK
Kieran

Hi there again,

The ZTX384 / 214 combination is often known as a complementary or series feedback pair.

A hunt through some electronics books should help you understand it, it's a little more complex than just a follower, owing to the feedback, but it's not too difficult.

It offers exceptionally good linearity for a simple circuit.

Jam's comments are interesting - may take a look at this.

Andy
 
Andy,

My mistake Andy you are correct. A four transistor cross-coupled buffer would work in the circuit which to me is still a better choice. My favourite is still the complemantary j-fet pair, partly due to simplicity and ease of offset correction.

I find complementary circuits work better than single ended ones when driving loads like filters (equal pull up and pull down), if a single ended buffer is used it has to be idled at higher currents to avoid asymetry.

Jam
 
Hello Geoff , Grey , Janne , Jam , Andy ,

Ok I have studied the drawings and can see the complementary feedback pair (ztx384/214)with the other two and the 12k and 68r res as a contant current source.

Just to pause there.....
The CFP I have found notes on but the constant current source in that configuration I can't find anything.Can someone fill in the gap please???

There are three 6dB/Octave filters making 18dB/Octave.

At the input and output there are a 10uF cap and 100k Ohm res in a filter looking arangment. Are they??

At the input the two 330k res are a pot div.
What is the 4.7k Ohm/470pF filter for at the input (after the pot div)?????

The neg feedback is not clear to me because it looks like 3 filters in sucsesion but with the middle one not to 0v but as feedback instead it has lost me.

Last bit now the 2.2k res(input)and 10k res(output) before the ztx384 of the CFP. Why.And can Geoff or someone send me the spiced version of the schematics or EWB version Please.

Kieran
 
Ins and outs are coupled with a cap: That is necessary, because the circuit runs with a single supply, you have to prevent a DC offset. 100k is to connect the cap to ground, not to leave it "open" to the outside world for several reasons.
The two 330k resistors set the bias point for the input cap - appr. halve voltage - and the reference voltage point for the input transistor.
All these do not have anything to with filters.
The actual input filter is the 4k7 / 470pf RC network - this is an essential part of naim design, I suppose.

The two constant current sources in the filter circuit are formed with two NPN transistors (ZTX384) and two resistors (47k and 68R).

Neill McBrides schematic drawing is actually a bit ODD and confusing.
Please check out his preamp drawings, than everything will be clear for you immediatly.


best whishes
Klaus
 
The 2k2 res. is part of the input circuit and the 10k is the input resistor for the output CFP. It is a part of the network already.
The filters are, as Geoff already mentioned, a passive 6db and an active 12db filter together. The output rail is feed back to the filter.
But I actually just follow the circuit paths with astonishment. These are very simple and very clever engineered circuits.
Neil McBride draw the schematics following the orginal PCB design, which Naim used for all crossover filters.

Klaus