2. amps-preamps
  3. 1258427381

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?
jpbach
newbee
4,794 posts
 
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.
gregm
3,411 posts
 
Hi
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
almarg's avatar
almarg
9,670 posts
 
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.

Regards,
-- Al
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