Tape in + tape monitor = passive preamp?

If you plug a line-level source into a preamp's "tape in" jacks, then flip the "tape monitor" switch, do you essentially get a passive preamp?

I ran across a post on the AudioCircle Bryston forum claiming that the "best input" on a Bryston BP26 was the tape in (or, as Bryston calls it, "tape from"). Here's the url, and the claim appears in the two entries by "werd": http://www.audiocircle.com/index.php?topic=69427.0

I checked the schematic for my B60R on the Bryston website to see if the same would apply to its preamp section. Sure enough, it does (if I read the diagram correctly). From the "tape from" input, the signal goes almost directly to the balance and volume controls; while the signal from any of the four main inputs goes through the selector switch and a multi-step circuit (pre-amplification, I assume) before reaching the balance and volume controls.

I decided to hear for myself how using the "tape from" input would affect the sound of my system. My initial impressions are that the sound is more detailed, primarily in the upper frequencies, but that it lacks body and bass. Switching back to the CD input results in a loss of HF detail but a more balanced sound.

I'm going to try living with the system in the "tape from" set up for a while. I like the extra detail, and maybe I'll get used to the frequency balance.

Has anyone else experimented with this technique? Using other brands of gear? Any advice on reclaiming the bass?
I think not. The hall mark of a passive pre-amp is the obvious. It can attenuate a line level signal but it cannot amplify it. In your set up you still have a pre-amp amplification circuit between your source input and the amp.

An anp section of a pre-amp is always at full gain (as seen by the amp) and the signal from the source is attenuated from full gain to zero gain. So you may hear a 'cleaner' signal using a tape monitor input for the reasons you outlined but you would not have a passive line stage as the signal is still passing thru the pre-amp's amplyfing stage.

There are a few pre-amps out there that do allow you to switch the amp output from active to passive, but not many. I believe Adcom was one.

I don't have a reason for your perception of a loss of bass. Only WAG's. Perhaps someone else can comment on this.
Any advice on reclaiming the bass?
Probably none. It seems that there is an impedance mismatch between your pre's output & the amp's input.
In other words, the pre is NOT outputting enough energy to drive the amp properly, hence you lose (energy in the) lower frequencies.

One thing you might try is a "hi-output" / "muscular" source. Ultimately it's the source driving your amplifier, via the pre's passive circuit. Regards
I took a look at the schematic for your B60R integrated amplifier.

First, along the lines of Newbee's comment, there is an active amplifier stage between the tape input and the input to the power amplifier section (or, correspondingly, between the tape input jacks and the preamp output jacks). So it cannot be considered to be a passive preamp, and more generally I don't know that it would be particularly meaningful to speak of the preamp section of an integrated amp as being passive even if that amplifier stage were not present.

The reason you are losing bass is that the input impedance at the tape inputs is very low, and your source component probably has an output impedance which rises at low frequencies (presumably due to the presence of a coupling capacitor at its output).

The input impedance of the regular line-level inputs appears to be close to 50K (50,000 ohms). The input impedance of the tape inputs will vary between 5K and 10K depending on the setting of the balance control. With the balance control set at mid-point, it will be 8.33K, according to my calculations. That is a very low value, which can only be driven properly by a source component having either very low output impedance, or an output impedance that varies very little with frequency (which most components are unlikely to have).

Unless detailed information is available on the output impedance of the source component, including its variations with frequency, and unless that information indicates a low output impedance at all frequencies (which your sonic observations suggest will not be the case), my recommendation would be to go back to the regular line-level inputs.

-- Al
One further thought: I would speculate that the reason you are sensing increased detail using the tape inputs may simply be that the attenuation of the low end is leading you listen at a higher volume level in that configuration.

-- Al
In the last sentence of my previous post, that should obviously be "leading you to listen at a higher volume level," not "leading you listen ..."

-- Al
Thanks to everyone for your comments. Checking the B60R schematic again, I see that there is additional circuitry between the Balance and Volume knobs and the amplifier stage. (Doh! -- I should have seen that myself.) That clearly makes the "tape from" path more than passive.

I guess that werd's idea was that "tape from" is a more direct path than the main inputs: less circuitry = cleaner. But in this case the low impedance seems to alter the frequency balance.

I think Almarg might have the answer for that problem. My CDP is a Bryston BCD-1. According to the measurements in Stereophile, output impedence is 74 ohms at high and middle frequencies, rising to 106.5 ohms at 20 Hz. Almarg, would that change in impedance be enough to result in rolled-off lower frequencies?

I'm curious about Gregm's advice on using a high output source. I was using the single-ended outputs from the BCD-1, which Stereophile measured at 2.38 volts. If I switched to the balanced outputs, which measured at 4.76 volts (and "exactly twice" the impedances), would that produce better results?

As for perceived increase in detail, I was very careful not to adjust the volume when switching from one set up to the next. However that does not eliminate the possibility that the "tape from" path might produce more decibels at the same volume setting.
I too use a BCD-1 as my cdp. Great minds think alike!

I use it's balanced outputs into the balanced inputs of a Classe CP-60 preamp, and I've been very pleased with it, listening mainly to classical music.

But I'm surprised to hear that is what you are using, because I don't think that the low frequency rise in output impedance from 74 to 106.5 ohms would be audibly significant, even into a 5K input impedance, which appears to be the worst case possibility for your B60R's tape inputs. So I'm not sure how to explain the bass loss, unless the output circuit of the BCD-1 is just uncomfortable in some way driving that low an impedance.

Re the comparisons being level matched, it's hard to tell from the schematic what the gain of the input buffer stage is, but it seems conceivable to me that it could be a bit less than 1 (resulting in a slightly lower volume at the same settings than from the tape input).

I don't think that the B60R has balanced inputs, so you can't use the BCD-1's balanced outputs without an adapter, which is not a good idea to do generally on outputs (most adapters short the inverted signal on pin 3 to ground). And it wouldn't help anyway because the adapter would simply route the non-inverted output to the amp's unbalanced input, the non-inverted output presumably having essentially the same voltage and impedance characteristics as the single-ended output.

-- Al