05-02-2019, 11:50 PM
Simple way to check if log or lin:
1. Rotate pot to mechanical centre.
2. Measure resistance between wiper and the anticlockwise end of the track.
If the resistance measurement is about half of the value, then it's lin. If it's about 10% of the value, then it's log. If it's about 90% of the value, then it's anti-log (or - quite likely in my case - you mis-identifed the anticlockwise end
)
Most often a pot marked "A" is log (A for audio), but sometimes "A" means linear - it just depends on the manufacturer. So I measure them and label them accordingly.
There is a chance that Rotel designed the circuit to work with lin pots - as I said earlier, I haven't looked inside - if so, simply copy one of the many other circuits out there - they are much the same apart from a few component value changes. Testing and experimentation is to be expected in any project - I never attempt to copy something with the expectation of it working perfectly for my needs first time.
Regarding the speaker/box, I really doubt the cabinet material is making a significant contribution when you look at all the other factors at play here. There's an excellent BBC paper available on the R&D website where they compared wood types for cabinets if you fancy a bit of bedtime reading: http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf
Far more important is the TS parameters of the drive unit, and the size of the box, and whether it's sealed, or whether there's a tuned opening like a port (which could be just a simple hole rather than a length of pipe). I won't go into all that as you probably know about it all, but suffice to say, it's not difficult to measure the relevant TS parameters of the drive unit, and with the help of something like WinISD, model the performances of that driver in that cabinet. Just depends on how keen you're feeling.
If you're thinking about subbing in those 4" bass drivers, I'd just fit a single one. It might seem wrong given that you'd be able to fit several in there, but multiple drivers won't allow you to go any lower in the bass end. Yes, they would handle more power, but a single one is easily good for 20 or 30 watts. Also, if you put several drive units together, you get an effect called "lobing", which means the frequency response in the midrange-treble region will vary enormously as you move your head around relative to the cabinet. It's horrible.
At the risk of rambling, I ran into exactly this when I started helping out at the local arts centre. On each side, they have 2 cabinets (Martin Audio AQ10), each containing a 10" woofer and a horn tweeter. They are side-by-side (touching), and driven in parallel. As you walk along the rows of seats, you hear a completely different sound as a result. So I unplugged one on each side, and that effect has gone away completely. And one per side is more than loud enough, so the other cabs are just sitting there as "hot spares". Because I know a lot of people would have a hard time understanding/believing me, I took a whole series of measurements at each seat in a row, and have a written report standing by for whoever is first to notice that I've unplugged some of the speakers... I'd love to meet the saleman who recommended this setup!
Anyway, from what I've seen of those 4" drivers, they will definitely have a smoother response through the mid and will extend further at the treble end, and I wouldn't be surprised if they go lower in the bass - knowing that old radio speakers have very stiff suspension and a high Q as they are designed to be used in open cabinets - even quite large vintage speakers can resonate at more than 100Hz. The box is probably a bit on the large side for those 4" units, but that really won't make a huge difference as you only have a couple of watts to drive it. Seal the back and bung a bit of damping in there to stop reflections causing a midrange peak (which you've already heard). Be aware that a lot of foam has practically no effect acoustically - you need the right stuff. Alternatively, get some wadding from some old speakers - the white BAF is OK, but many alternatives exist.
In conclusion, the scope for experimentation is endless, and I would encourage anyone to play around for as long as they're having fun and learning stuff. If it's becoming boring or feels like a chore, then cut your losses and go with what you've achieved so far, but always consider re-visiting the design once you've listened to it over a few weeks or months. I really love playing around with speakers. I've got a design on the go here which has been brewing for months - every now and again I'll get the measurement mic out and try a new crossover idea, then listen to it while I'm working on other things for a few weeks, then repeat. Passive crossover design is just about one of the hardest things I've done - the circuits might look simple, but they are extremely complex in their operation - and I enjoy the challenge. Active speakers - where there's no interaction between the crossover filters and the complex impedance of the drive units - is too easy!
Enjoy
1. Rotate pot to mechanical centre.
2. Measure resistance between wiper and the anticlockwise end of the track.
If the resistance measurement is about half of the value, then it's lin. If it's about 10% of the value, then it's log. If it's about 90% of the value, then it's anti-log (or - quite likely in my case - you mis-identifed the anticlockwise end
)Most often a pot marked "A" is log (A for audio), but sometimes "A" means linear - it just depends on the manufacturer. So I measure them and label them accordingly.
There is a chance that Rotel designed the circuit to work with lin pots - as I said earlier, I haven't looked inside - if so, simply copy one of the many other circuits out there - they are much the same apart from a few component value changes. Testing and experimentation is to be expected in any project - I never attempt to copy something with the expectation of it working perfectly for my needs first time.
Regarding the speaker/box, I really doubt the cabinet material is making a significant contribution when you look at all the other factors at play here. There's an excellent BBC paper available on the R&D website where they compared wood types for cabinets if you fancy a bit of bedtime reading: http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf
Far more important is the TS parameters of the drive unit, and the size of the box, and whether it's sealed, or whether there's a tuned opening like a port (which could be just a simple hole rather than a length of pipe). I won't go into all that as you probably know about it all, but suffice to say, it's not difficult to measure the relevant TS parameters of the drive unit, and with the help of something like WinISD, model the performances of that driver in that cabinet. Just depends on how keen you're feeling.
If you're thinking about subbing in those 4" bass drivers, I'd just fit a single one. It might seem wrong given that you'd be able to fit several in there, but multiple drivers won't allow you to go any lower in the bass end. Yes, they would handle more power, but a single one is easily good for 20 or 30 watts. Also, if you put several drive units together, you get an effect called "lobing", which means the frequency response in the midrange-treble region will vary enormously as you move your head around relative to the cabinet. It's horrible.
At the risk of rambling, I ran into exactly this when I started helping out at the local arts centre. On each side, they have 2 cabinets (Martin Audio AQ10), each containing a 10" woofer and a horn tweeter. They are side-by-side (touching), and driven in parallel. As you walk along the rows of seats, you hear a completely different sound as a result. So I unplugged one on each side, and that effect has gone away completely. And one per side is more than loud enough, so the other cabs are just sitting there as "hot spares". Because I know a lot of people would have a hard time understanding/believing me, I took a whole series of measurements at each seat in a row, and have a written report standing by for whoever is first to notice that I've unplugged some of the speakers... I'd love to meet the saleman who recommended this setup!
Anyway, from what I've seen of those 4" drivers, they will definitely have a smoother response through the mid and will extend further at the treble end, and I wouldn't be surprised if they go lower in the bass - knowing that old radio speakers have very stiff suspension and a high Q as they are designed to be used in open cabinets - even quite large vintage speakers can resonate at more than 100Hz. The box is probably a bit on the large side for those 4" units, but that really won't make a huge difference as you only have a couple of watts to drive it. Seal the back and bung a bit of damping in there to stop reflections causing a midrange peak (which you've already heard). Be aware that a lot of foam has practically no effect acoustically - you need the right stuff. Alternatively, get some wadding from some old speakers - the white BAF is OK, but many alternatives exist.
In conclusion, the scope for experimentation is endless, and I would encourage anyone to play around for as long as they're having fun and learning stuff. If it's becoming boring or feels like a chore, then cut your losses and go with what you've achieved so far, but always consider re-visiting the design once you've listened to it over a few weeks or months. I really love playing around with speakers. I've got a design on the go here which has been brewing for months - every now and again I'll get the measurement mic out and try a new crossover idea, then listen to it while I'm working on other things for a few weeks, then repeat. Passive crossover design is just about one of the hardest things I've done - the circuits might look simple, but they are extremely complex in their operation - and I enjoy the challenge. Active speakers - where there's no interaction between the crossover filters and the complex impedance of the drive units - is too easy!
Enjoy
