Those Familiar With Old School ‘Tape Monitor’ Loops

[Mean & Green]

Can anyone explain this problem to me?

I have inserted a Schiit Loki tone control into my system via my NAD C326BEE integrated amps ‘tape monitor’ loop. I can use the Loki as I want with any source by simply engaging the monitor function on my amp. Now most of the time I do not require the Loki so leave it switched out of circuit, great right? Well… I have noticed significant audible distortion with my system when the Loki itself is switched off even though it shouldn’t be in circuit due to not activating the ‘tape monitor’ loop.

I have noticed this in the Loki instructions. (See attached image).

Leaving the Loki on even when not in use and even when ‘tape monitor’ is off or on, all is fine - but if I switch the Loki completely off I get distortion which gets worse if ‘tape monitor’ is activated, but distortion is still there even when ‘tape monitor’ is off. How can switching the Loki completely out of circuit via ‘tape monitor’ being off still have an impact?

I understand I can just switch the Loki on even when not using it, but I don’t see why I should have to when it should be completely bypassed via the ‘tape monitor’ switch.

Can anyone explain this to me? ‘Tape monitor’ on should include the Loki in the chain. ‘Tape monitor’ off should be completely bypassed so how can it have any effect whether the Loki itself is on or off? It makes no sense to me why any part of the Loki matters when ‘tape monitor’ is off.

 

[MaxwellsEq]

It might be that when off, the Loki has a low impedance which is loading the tape loop output buffer.

 

[Mean & Green]

It might be that when off, the Loki has a low impedance which is loading the tape loop output buffer.

Thanks, that makes sense actually.

It’s not a big deal, all I have to do is switch it on every time I use the system and switch it in and out as desired. I just wasn’t expecting any impact on the system when completely switched out, but yeah I guess it could be loading the tape loop when powered down.

Dunno why I didn’t think of that myself.

 

[MaxwellsEq]

Thanks, that makes sense actually.

It’s not a big deal, all I have to do is switch it on every time I use the system and switch it in and out as desired. I just wasn’t expecting any impact on the system when completely switched out, but yeah I guess it could be loading the tape loop when powered down.

Dunno why I didn’t think of that myself.

Another possible issue is that signal earth changes when on vs off.

But a tough buffer on the output of the tape loop should cope

 

[Blumlein 88]

It was a problem with many old preamps that if you had something in the tape loop which was off, the off device had an odd loading effect due to switched off output transistors. In those cases it was an off device connected to the tape loop that could cause distortion in a preamp which was on and tape loop not in use. So this is not the exact situation with this device, but it may be something similar depending upon how it is wired. In any case just follow instructions and leave it on.

 

[Mean & Green]

It was a problem with many old preamps that if you had something in the tape loop which was off, the off device had an odd loading effect due to switch off output transistors. In those cases it was an off device connected to the tape loop that could cause distortion in a preamp which was on and tape loop not in use. So this is not the exact situation with this device, but it may be something similar depending upon how it is wired. In any case just follow instructions and leave it on.

Interesting I’ve never encountered this or heard of it previously. Every day is a school day!

 

[MaxwellsEq]

Interesting I’ve never encountered this or heard of it previously. Every day is a school day!

Very common in the 70s and 80s when we had cassette decks. Some tape machines behaved well when switched off (perhaps using a relay on the input).

 

[Mean & Green]

Very common in the 70s and 80s when we had cassette decks. Some tape machines behaved well when switched off (perhaps using a relay on the input).

I used to have cassette decks with other amps, but was completely unaware of this potential issue.

I also used to have a CD-R connected to this very same tape loop I am using now, again with no issues. It obviously varies depending on the components in the chain depending on design.

I’ve learned something new and now I’m aware of it I can accommodate. At least I know nothing is malfunctioning.

 

[Blumlein 88]

Very common in the 70s and 80s when we had cassette decks. Some tape machines behaved well when switched off (perhaps using a relay on the input).

Reel to reels too. Cannot remember if it was my Revox B77 or another RTR, but one of my reel machines was a problem with some simpler preamps like a Conrad-Johnson PV5. I think the Adcom pre/tuner back then had a buffered tape loop. So it was okay.

 

[mhardy6647]

The funny thing is that some, perhaps even many, tape loops were simply pass-throughs. I'll spare y'all the backstory

Spoiler

Kenwood 'control amplifier' troubles

OK, here's a problem. Our town Teen Center received the donation of a more or less complete 1980's vintage Kenwood stereo component/rack (NOT all in one) system. This is comprised of a power amp (billed as 125 wpc and pretty respectably hefty), a 'control amp', a dual cassette deck and...

www.audiokarma.org

but I spent some time trying to understand the operation of a very cheap/low end Kenwood preamp (KC-206). One thing I discovered: it would always pass signal through its tape loop inputs to outputs. Didn't have to be turned on or plugged in.