Anyone ever use a Jensen input transformer in a preamp? If so, where should it be located? The Jensen site is unclear about source impedance requirements. Assuming source impedance should be small, it would seem that the transformer should be placed before the volume control. This way, the volume control could always provide a load impedance greater than the required 10K ohms. However, the load would vary with volume selection.
If, on the other hand, the transformer is placed between the volume control and the amp, the idea load of 10k could be had, and it wouldn't change. However, the source impedance would be higher and variable. My hunch is that the first layout would be preferable, but not sure of the tradeoffs. Anyone know? I'm not a big enough customer for Jensen to answer my question.
If, on the other hand, the transformer is placed between the volume control and the amp, the idea load of 10k could be had, and it wouldn't change. However, the source impedance would be higher and variable. My hunch is that the first layout would be preferable, but not sure of the tradeoffs. Anyone know? I'm not a big enough customer for Jensen to answer my question.
pooge said:
That depends on the type of volume control you're using. Unless you're using an attenuator which uses a fixed series resistance and a switched shunt resistance, the load will stay the same unless the input impedance to the preamp itself is unusually low.
And you wouldn't want the pot to provide a load greater than 10k, at least not for the 11P-1 and 11P-1HPC if you want to avoid using the RC damping network. These two are both ideally loaded with a 10k resistive load so using either of these with a 10k pot or attenuator would be ideal.
You might also want to consider the 11P4-1. It's ideally resistively loaded at 20k and also provides a wee bit of voltage gain compared to the 11P-1's 3dB loss.
Again, the ideal load can be had using a 10k pot or attenuator. As long as it's not of the fixed series/adjustable shunt type, it will present a 10k load to the transformer.
Why do you say this? Have you asked them already and they haven't answered?
se
I am using a fixed series/switched shunt. That is why I'm having questions. I anticipated having to use the RC damper if the transformer is placed before the volume control.
The control will always be above 10K load.
As I said, I can get an ideal load if placed after the lineamp, but I'm unsure of the variable source impedance from the volume control. Can't find any info on source impedance issues.
I do not need any more gain with horn loudspeakers, but the 20K load of the 11P4 looks interesting.
I wrote them over a week ago, and got no reply.
The control will always be above 10K load.
As I said, I can get an ideal load if placed after the lineamp, but I'm unsure of the variable source impedance from the volume control. Can't find any info on source impedance issues.
I do not need any more gain with horn loudspeakers, but the 20K load of the 11P4 looks interesting.
I wrote them over a week ago, and got no reply.
pooge said:
Ah, ok.
It's not so much the variability of the source impedance as it is how high your source impedance is. What's the value of the series resistor in your volume control?
Basically, increasing source impedance will result in increased distortion (primarily at the lowest frequencies), reduced low frequency bandwidth and higher losses.
I think you'd be better off with the transformer feeding your volume control. That's really where you'd want it anyway. If you put it after your volume control, then you lose the advantage of ground isolation.
Yes. I mentioned the 11P4-1 not so much for its voltage gain (which is just 1.5dB) but rather for its reduced voltage loss. With the volume control you're using now, you've already got a fair amount of loss built-in. No need to add to it if you don't have to.
Strange. In my experience (and I'm hardly their biggest customer) they've always been very responsive. And not just to me but to those I've sent their way who weren't customers at all.
If you'd like, EMail me the name and EMail address you used to contact them and I'll look into it for you.
se
I looked at the 11P4-1. The specs aren't as good. Even with a 20K load, the input impedance of the transformer is not as high as with 11P1 with a 10K load. Also, the CMMR specs aren't as good.
I deduced from the specs that a higher source impedance degrades CMRR. Thus, it would appear to be better to place xformer before volume control to benefit from the low output impedance of a source, and use the RC damper.
I made a stepped volume control designed with a 12K series impedance (before contemplating a xformer). This can be either single-ended or balanced. I will probably use two 6K series resistors on either side of the switched resistor to feed a balanced lineamp. (However, I read somewhere that the secondary of the transformer is not balanced, so I don't know if this would be that useful. Perhaps that is why all the application notes show the secondary grounded at one end.)
BTW, what is your take on the transparency of these transformers?
I deduced from the specs that a higher source impedance degrades CMRR. Thus, it would appear to be better to place xformer before volume control to benefit from the low output impedance of a source, and use the RC damper.
I made a stepped volume control designed with a 12K series impedance (before contemplating a xformer). This can be either single-ended or balanced. I will probably use two 6K series resistors on either side of the switched resistor to feed a balanced lineamp. (However, I read somewhere that the secondary of the transformer is not balanced, so I don't know if this would be that useful. Perhaps that is why all the application notes show the secondary grounded at one end.)
BTW, what is your take on the transparency of these transformers?
pooge said:
No, the absolute specs aren't as good. But then they're just specs and won't necessarily tell you how it will sound.
Yes. Though perhaps more importantly, you lose the galvanic isolation between components that transformers can afford.
The secondaries aren't balanced so if you want to feed a balanced input with them, you'll need to create a pseudo center tap using a pair of resistors for the load. A pair of 5k resistors for the 11P-1. Then use the node between the two resistors as the ground reference for the shield/case as well as the amplifier's reference ground.
Let's just say that I've been using them for nearly 20 years and have been very happy with them.
se
fdegrove said:There not unbalanced unless the secundary is grounded on one end...in that scenario the xformer serves a balun to the next stage which is often ahem...SE.
The secondaries of input transformers such as the 11P-1 are not balanced. That's due to the winding techniques used to get the level of performance out of them that they offer, primarily keeping the winding capacitance as low as possible in order to achieve 100kHz bandwidths.
Yes. But the neat thing about a good quality input transformer is that they can give you outstanding common-mode rejection even if they're fed from a wholly unbalanced source. The 11P-1 for example will give you about 100dB of common-mode rejection when fed from an unbalanced source. Which is far greater than most electronically balanced inputs will give you even when they're fed from a balanced source.
For an electronically balanced input to get the same performance from a balanced source that a transformer can give you from an unbalanced source, you pretty much have to match the source impedances to a rather high degree of precision.
se
Are you saying the 11P4s sound better?
Not sure I understand this comment. How does source impedance affect galvanic isolation? Since this is done by the transformer, what does the source impedance matter?
Although I can configure my lineamp and/or volume control to be balanced or unbalanced, I'm wondering if it's worth the bother at that point. I'm primarily looking to break ground loops (damn those horns are sensitive suckers!), and I doubt there's much to worry about in the short distance between the volume control and lineamp.
Exactly! And the fact that you can connect a balanced or unbalanced input without any further fuss. And Jensen shows an easy way of impedance balancing the output of an unbalanced source in Figure 4 of AN-003. Although I am confused by the different recommendations for the ground connection of the case in AS002 and AS089. In one, it is connected to the output signal ground. In the other, it is connected to chassis ground. All is made more confusing in that the case is conductive, and there is no recommendation other than the warning that a ground loop may be created with the case lead if the case is not insulated from ground. The two schematics appear to be for somewhat different applications, though. One appears to be an outboard balanced to unbalanced converter, while the other appears to be inboard.
For inboard, like I will be using, I guess you just leave the white wire disconnected if the case is conductively mounted to the chassis, such as with the new case style that can more easily mount the transformer in a hole in the back of an amp case, with the wiring easily feeding through the mounting hole to the connectors. (This conductive mounting doesn't square with AS089, but all in all, probably doesn't matter.)
I'm primarily looking to break ground loops (damn those horns are sensitive suckers!), and I doubt there's much to worry about in the short distance between the volume control and lineamp.
Put the Tx immediately after the selector switch then VC.
I also run horns and use a similar sysuem with a TVC and have no hum or noise.
Put the Tx immediately after the selector switch then VC.
I also run horns and use a similar sysuem with a TVC and have no hum or noise.
fdegrove said:
Why do you say that? If you need to feed a balanced input, it's a simple matter of splitting the load between two resistors instead of one.
I'll pass that on to them. What exactly is a "real transformer" though so they'll know what to shoot for?
Then why did you bother to post?
se
pooge said:
No. Just saying that raw specs won't always tell you whether you'll like something or not. If they did, we'd all run out and buy Halcro amps.
Sorry, brain fart. I was envisioning the transformer after the attenuator scheme with the attenuator's ground tied to the input reference ground. The attenuator could be connected wholly to the primary and you'd still have your isolation.
Well, they way I've looked at it is that something like the 11P-1 is already arguably the best differential input you can get so unless you're going to have some unusually long run between the transformer's output and the next stage's input, there's not much point in the added complexity of another differential input.
No, that's not what they're doing in AN-003. They're just feeding the 11P-1's output straight into a balanced input. They're not balancing the output impedance there.
Check out AS-060 instead.
Yes. Not sure what their reasoning is there. I'll ask Dale to get their definitive answer.
That'd work as long as the chassis is grounded. Otherwise it would allow capacitive coupling to the transformer. Personally, I'd keep the case isolated from the chassis and run the case lead along with the shield lead to your ground point.
If you'd like, I could send you some of the custom die cut IsoDamp washers that I use (which would also help out as far as vibration goes). Then just use some fiber or nylon shoulder washers to isolate the screws (you'd need a bit longer ones) from the chassis.
se
I was referring to the trick for balancing the output impedance of the *source* in Figure 4, Not the output of the transformer. It is said to improve CMRR, but not by a whole lot. Still, easy to do.
On second thought, the transformer in a preamp would be inside, after the selector, as stated in another post. I think the new case would be better usable in a power amp. I was concidering something like a capacitor clamp for the transformer in a preamp, but havn't found one that small yet.
Not sure what these are and/or how you use them, but I'd be interested in benefitting from your experience.
pooge said:
Figure 4? Sorry, you've lost me. What Figure 4? Thought we were talking about AS-003?
Jensen sells the 11P-1 with a little L-bracket that you can use for mounting as well.
They're just a gasket I had made out of 1/16" IsoDamp damping material. They're the same diameter as the bottom of the 11P-1 and have two cutouts for the holes where the lead wires come out and two holes for the screw holes. They're sandwiched between the transformer and the mounting surface and the transformer's screwed down as you would otherwise.
They provide both electrical insulation and vibration control.
I manufacture a product which uses the 11P-1s and I had these made for mounting them to the circuit board. I've a couple thousand of them so if you'd like a few pair to play with, just EMail me a mailing address and I'll stuff 'em in an envelope and mail 'em out to you.
se
pooge said:
Ah, ok.
They're not balancing the source in AN-003. The source is assumed to be balanced already.
What they're doing in AN-003 is turning an existing electronically balanced input into an unbalanced input (shorting pins 1 and 3) and using the 11P-1 to provide much better common-mode rejection from the balanced source than the electronically balanced input offers.
This is why I've never bothered to use an 11P-1 to feed a balanced input.
You're welcome. They'll go out in tomorrow's mail.
No, I don't sell the bare transformers. And I don't buy them in large enough quantities that I could sell them for any sizeable discount. The prices listed on their website are the prices you'd pay whether you're a DIYer or a manufacturer. They don't have any separate wholesale price for their raw transformers (as opposed to their IsoMax products) except perhaps to their foreign distributors.
se
Whoops! That's figure 2.4, where they "balance" the output of an unbalanced source to feed the input of the transformer. They just duplicate the impedance of the + side of the unbalanced source on the ground output. This creates the balanced impedance bridge for common mode ground noise source between chassis to help further increase CMRR over the transformer alone.
pooge said:
D'OH! Ok, I'm with you now. I was thinking AN-003 and AS-003 were one and the same. Sorry. I've got all this stuff in a big binder from Jensen and always associated the application notes with their titles and the application schematics by their number.
No wonder I got thrown off with your references to figure numbers. Don't see a dunce cap smiley so I'll just have to make due with this one
Just imagine that it's a dunce cap rather than a wizard's hat. Ok, yeah. That will convert an unbalanced output to a balanced output alright. And yeah, with something like the 11P-1, you don't really get a huge advantage in terms of common-mode rejection and you just end up with a more complex output circuit.
se
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