My tastes have evolved some over the years.

pcrecord, post: 461167, member: 46460 wrote: I'm changing the tubes on one of them in a few weeks, I'll probably do a video on it too.. Stay tuned for that !

Oh yeeeeeaaaaaa

pcrecord, post: 461167, member: 46460 wrote: But this experience with the attenuator re-confirmed to me that the ISA is all about Clean sound, no noise, no distortion.. but still offers this nice transformer sound.

Totally. From what i understand they chose the lundhal xformer, specifically because it's one of the cleanest xformers available. Plus if you consider the isa was made for a console, with eq and compressor, it makes sense that its cleaner with tons of headroom. The ISA is one of those pres that just sounds right imho.

I would be curious to hear the attenuator in series, just to hear how the isa saturates.

kmetal, post: 461171, member: 37533 wrote: I would be curious to hear the attenuator in series, just to hear how the isa saturates.

Sufficient attenuation between the output of a pre-amp and a following ADC allows the ADC to capture either the effect of transformer saturation or the onset of clipping, whichever is appropriate for the pre-amp's output architecture.

I've done a bit more digging in the Focusrite literature, and, from what I can tell, the ISA One, Two, 428 and 828 are based on a common design that has input transformers only and no output transformers. The information is well hidden, but with these models, it looks as though the output will clip.

Boswell, post: 460601, member: 29034 wrote: Here you are:

Hey Boswell, would you be kind enough to calculate what the values for a 40db would be?
It's a question Faraz Abtahi&lc=UgzYWiXPVhZb9HSBUcp4AaABAg'">il asked on my youtube channel.

This is for the U-pad in your balanced attenuator, right? Well, 40dB is easy, as it's 100x in voltage and hence ratios. To 1%, you can use a shunt of 2R and series of 100R. Series resistors of 10K Ohms and a shunt of 200 Ohms are readily available values, and would work OK in this configuration (where OK is not a short circuit). Use two 100 Ohms in series if you can't get 200 Ohms, but if you are considering that, then 4.7K series and 2x 47 Ohm shunt would work marginally better over the range of other likely circuit impedances.

Boswell, post: 462289, member: 29034 wrote: This is for the U-pad in your balanced attenuator, right? Well, 40dB is easy, as it's 100x in voltage and hence ratios. To 1%, you can use a shunt of 2R and series of 100R. Series resistors of 10K Ohms and a shunt of 200 Ohms are readily available values, and would work OK in this configuration (where OK is not a short circuit). Use two 100 Ohms in series if you can't get 200 Ohms, but if you are considering that, then 4.7K series and 2x 47 Ohm shunt would work marginally better over the range of other likely circuit impedances.

Thanks a lot, I'll relay that answer..

Hello 5 years later,

I'm a new member and would like to bring this topic back for an attenuator I'm going to build. The objective is exactly the same but I would like to have 3 settings (2 different attenuation options plus a bypass) so as Boswell said I would need a 3 steps toggle switch (on - off - on). I've spent the past week researching and trying to understand switches better, but I'm still fairly confused on which type of switch would be needed for this. DPDT doesn't seem to allow for enough options to get 3 different settings, but (if my understanding is correct) a 3PDT also wouldn't do the job as I would probably need 3 throws? Am I correct or completely off the rails?

Thank you

Hi Giulio,

You can either do this simplistically or precisely. Which you do depends on the output impedances of the devices that are likely to drive the attenuator and  the input impedances of the devices that the attenuator has to drive.

If you can say that the driving impedances are 100 Ohm or less and that the driven impedances are 10K Ohm or more, then the simple method is all you need to get attenuation accuracies of about 1dB. However, if you are working, for example, in a 600 Ohm environment, then you need a much more precise configuration.

You also don't say whether you want more than one signal channel, and if so, do you need the attenuations to switch in channel pairs (e.g. as in a stereo attenuator).

The simple fixed balanced attenuator is a U-pad. This has two equal resistors (R1) connected to the + and - inputs, and a single different resistor (R2) connected across the free ends of the R1 resistors. The + and - output signals are taken from across R2. If you sketch this, it's easy to see that if the input is balanced, the mid-point of R2 will be at zero volts. This is why R2 values are often given as 2xR2. If you put a vanity mirror on edge along the centre line, you will see the symmetry.

To turn this into a switchable atenuator, simply use a SPDT switch to select one of two different resistors for R2. If the switch has a centre-off position, then selecting that gives an approximately zero attenuation setting. Make sure you always take the output from the ends of the two R1s, irrespective of the switch setting. Note that a stereo (2-channel) version can be made using a single DPDT switch.

Attenuation ratios are easy to calculate. If R2 represents half the physical value of the actual shunt resistor, then the attenuation is approximately R2/(R1+R2). The tricky bit is to choose resistor values that allow your different attenuations without too much variability due to the change in attenuator output resistance driving the unchanged load.

As a simple example, a 6dB -12dB attenuation (2x or 4x voltage drop) could be done with a pair of 1.5K Ohm input resistors (R1) and switch selection of 3K or 1K shunt resistors (R2). Each leg of the circuit then would have either 1.5K/(1.5K + 1.5K) = 1/2 or 0.5K/(0.5K + 1.5K) = 1/4.

Good luck!